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A PHYSIOLOGICAL STUDY OF THE CLI 

MATIC CONDITIONS OF MARYLAND 

AS MEASURED BY PLANT 

GROWTH 

(A second contribution from data obtained under the auspices of the 
Maryland State Weather Service, in 1914) 



Dissertation submitted to the Board of University 

Studies of the Johns Hopkins University in 

conformity with the requirements 

for the degree of Doctor of 

Philosophy 



By 
F. MERRILL HILDEBRANDT 






in 



BALTIMORE 
June, 1917 



IRbprinted from Physiological Reseaeches, 2: 341-405. 19211 



A PHYSIOLOGICAL STUDY OF THE CLI 

MATIC CONDITIONS OF MARYLAND 

AS MEASURED BY PLANT 

GROWTH 

(A second contribution from data obtained under the auspices of the 
Maryland State Weather Service, in 1914) 



Dissertation submitted to the Board of University 

Studies of the Johns Hopkins University in 

conformity with the requirements 

for the degree of Doctor of 

Philosophy 



By 
MERRILL HILDEBRANDT 



BALTIMORE 

June, 1917 

I&EPRINTED FROM PHYSIOLOGICAL RESEARCHES, 2: 341-405. 1921] 



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A PHYSIOLOGICAL STUDY OF THE CLIMATIC CONDITIONS OF 
MARYLAND, AS MEASURED BY PLANT GROWTH 1 

A Second Contribution from Data Obtained Under, the Auspices of 
the Maryland State Weather Service, in 1914 

F. MERRILL HILDEBRANDT 

ABSTRACT 2 

The present paper presents the results obtained from a study of a series of obser- 
vations on the climatic complexes for nine different stations in Maryland for the sum- 
mer of 1914, as the effectiveness of each complex was automatically integrated by soy- 
bean plants grown for a period of 4 weeks from the seed, new seeds being planted every 
2 weeks. Corresponding instrumental observations were also studied. 

The field work was carried out by Dr. Forman T. McLean, under the joint auspices 
of the Maryland State Weather Service and the Laboratory of Plant physiology of the 
Johns Hopkins University. McLean 3 has presented an account of the plan and methods 
by which the observational data were obtained and he also made a thorough study of 
the growth data for soy-bean, for the stations at Oakland and Easton, his main results 
and conclusions having been set forth in his publications. 

The study with which the present paper deals involved all the climatic and soy-bean 
data obtained by McLean; it thus included data for nine different localities well dis- 
tributed throughout the state. The plants for all stations and for all periods were 
treated practically alike, excepting for the climatic conditions of the several localities. 
The same soil was used for all cultures. The values obtained from the plant measure- 
ments, made after 2 and after 4 weeks of growth, therefore constitute comparative and 
quantitative descriptions of the total influence exerted by all the climatic conditions 
upon the plants, and they exhibit the seasonal march of this climatic resultant in terms 
of the responses of the different sets of cultures. The plant measurements themselves, 
and the values derived from them, form perhaps the most important contribution in 
the present paper. The graphs of these values depict the seasonal march of each cli- 
matic complex, not after the manner of the instrumental readings usually employed in 
the study of climate, but as measures of the effectiveness of the climatic complex to 
favor or retard the growth of the standard plant, the latter being employed as a "living 
instrument" or indicator of climatic effectiveness. 4 

The plant measurements employed are: (1) stem height, (2) leaf area, (3) leaf- 
product (length multiplied by width), (4) dry weight. The climatic values used are: 
(1) air temperature, (2) the evaporating power of the air, (3) sunlight intensity and 
duration. Values to represent the temperature efficiency for growth are derived from 
the temperature records by means of the Livingston physiological temperature indices 



1 Botanical contribution from the Johns Hopkins Hospital University, no. 60. This work was practically 
completed in 1917, but war conditions delayed its publication. 

2 This abstract was preprinted, without change, from these types and was issued as Physiological Researches 
Preliminary Abstracts, vol. 2, no. 8, May, 1921. 

3 McLean, Forman T. A preliminary study of climatic conditions in Maryland, as related to plant growth 
Physiol. Res. 2: 129-208. 1917. A brief account had appeared earlier: — Idem. Relation of climate to plant 
growth in Maryland. Monthly Weather Rev. 43: 65-72. 1915. 

* Livingston, B. E-, and McLean, F. T. A living climatological instrument. Science 43: 362. 1916. 

341 

PHYSIOLOGICAL RESEARCHES, VOL. 2, NO. 8 
SERIAL NO. 18, MAY, 1921 



342 F. Mebbill Hildebeandt 

All of the measurements, both plant and climatic, are expressed relatively, so that they 
may be compared for different stations and for different periods. 

Considering the soy-bean plant (as here employed) as a standard plant or indicator 
for the measurement- of climatic efficiency to produce plant growth, it appears that, 
for the season of 1914, the climatic complexes for some culture periods were more efficient 
in this sense than might be expected from an attempt to interpret the corresponding 
climatic data, while the complexes for other periods were actually less efficient to pro- 
duce soy-bean growth than might be surmised from the corresponding climatic values. 
No successful method has yet been brought forward by which the value of a climatic 
complex, to produce growth in any plant form, may be deduced from instrumental 
data, and the plant measurements of this study furnish a means by which climatic 
efficiency as a whole may be directly compared for different periods and for different 
stations. The seasonal means derived from the plant data give relatively low efficiency 
values for the climatic complexes of the three western stations (Oakland, Chewsville 
and Monrovia), high values for the complexes of Baltimore and Darlington, and inter- 
mediate values for the complexes of the remaining stations (College, Coleman, Easton, 
and Princess Anne). 

If indices for total seasonal climatic efficiency are derived by multiplying the seasonal 
average growth rate per day by the normal length (days) of the growing season for the 
station in question, these indices have the following values for the several stations: 
Oakland, 9009; Chewsville, 12480; College, 16867; Easton, 17688; Princess Anne, 19005; 
Coleman, 21115; Darlington, 236SS; Baltimore, 25422. In considering these relative 
climatic indices it should be emphasized that the data of this study do not involve pre- 
cipitation as an influential climatic feature; the culture plants were automatically 
irrigated so that they never suffered from lack of soil moisture. 

A study was made of the interrelations of the different kinds of plant measurements, 
dealing therefore with some aspects of growth correlation in soy-bean. It appears 
that the rate of stem elongation was greater than the rate of leaf expansion when both 
were relatively small, while the former rate was the smaller of the two when both were 
relatively large. The rate of production of dry weight appears to have been nearly pro- 
portional to the rate of increase of leaf surface ; the relative values of these two growth 
criteria are generally about equal numerically. 

The statements just made apply to the data for plants quite openly exposed, but 
some observations on cultures somewhat protected above by glass were available, and 
also a set of observations on cultures in forest at Baltimore. These all indicate that 
the height rate was relatively greater than the rate of leaf expansion for these more or 
less shaded conditions, while the rate of dry-weight production was smaller than the 
corresponding rate of leaf expansion. The outcome of this part of the study may throw 
some light on the general problem as to what sort of plant measurements may be best 
suited to quantitative comparisons of the efficiencies of different climatic complexes. 
An interesting, and probably valuable result of this study is that the calculated leaf- 
product (length times breadth, which can be obtained without injury to the plants, if 
that is desirable) is generally proportional to the leaf area; of course for these soy-bean 
plants. 

The climatic data themselves showed a pronounced general agreement between the 
graph for sunlight and the corresponding one for evaporation (standardized white 
cylindrical porous-cup atmometer), this being probably due to the relatively great 
importance of solar radiation in determining the evaporation rate. The climatic 
values indicate a general seasonal march, which is very evident for temperature, less 
so for sunlight and rather obscure for evaporation. For details regarding the climatic 
values, as well as for the plant values, reference must be made to the tables and graphs 
and to the text of the paper. 



Climatic Conditions op Maryland 343 

CONTENTS 

Introduction 344 

The observational data and the averages derived from them 

The plant measurements 349 

The climatic measurements 352 

Temperature 353 

Light 357 

Evaporation 359 

Results and discussion 

Introductory 360 

Results from stations in the open 
The 2-week values 
The 2-week plant data for stations in the open 

Correlations between the 2-week plant graphs '. . 371 

Trends of the 2-week plant values and their seasonal ranges for the sev- 
eral stations 376 

The 2-week climatic data for stations in the open 377 

The 2-week temperature data 378 

Chewsville and Monrovia 378 

Baltimore, Darlington and Coleman 379 

Easton and Princess. Anne 379 

College 379 

Oakland 379 

The generalized graph 380 

Light and the evaporating power of the air, 2-week data 380 

Variability of temperature and evaporation values 382 

Correlation of the 2-week plant and climatic values 383 

The 4-week values 385 

The 4-week plant data for stations in the open 386 

The 4-week climatic data for stations in the open 390 

Results for the three covered stations 

Introductory 391 

The plant data, covered stations 391 

The Oakland covered station 393 

The Baltimore covered station 394 

The Easton covered station 394 

The climatic conditions, covered stations , 394 

Results for the Baltimore forest station 395 

The plant data as measures of the climatic efficiency for growth of the standard 
plants 

Introductory 396 

Seasonal averages of mean daily intensity values for the several stations. . . . 398 

Total seasonal efficiencies for the several stations 403 

General conclusion 405 



344 F. Merrill Hildebrandt 

INTRODUCTION 

During the summer of 1914 an elaborate investigation was undertaken by 
the Maryland State Weather Service in cooperation with the Laboratory of 
Plant Physiology of the Johns Hopkins University, with the object of ascer- 
taining some of the relations between climatic conditions and the growth of 
certain plants exposed at different stations in Maryland. Detailed informa- 
tion as to the growth of the plants used is of course necessary in such a study, 
as is also corresponding knowledge of those environmental conditions that 
are considered as climatic. The plant records were secured in this case by 
growing cultures of certain plants under the environmental conditions to be 
studied, and noting the amount of growth accomplished during definite peri- 
ods of time. In order that a corresponding series of measurements of some 
of the environmental conditions might be available for comparison with 
these growth measurements, the cultures were located at certain of the 
regular observation stations of the U. S. Weather Bureau, at nine different 
places in the state. The general plan of the study and a detailed considera- 
tion of the methods used has already been published by McLean, 5 who did 
all of the field work personally. The original data dealt with in the present 
paper were secured from McLeans' records, and it will be necessary to give 
here only so much description of the ways in which these measurements were 
obtained as is needed to render their use in the present publication intelligible. 
The following description is taken mainly from McLean's paper, which also 
deals with the growth of soy-bean plants, but for only two of the stations, 
Easton and Oakland. The present paper gives the main results for soy-bean 
plants for all of the nine stations, together with some attempts at interpreta- 
tion. 

This study has been carried out partly through financial aid furnished by 
the Maryland State Weather Service. It was suggested by Prof. B. E. Liv- 
ingston and carried out under his direction. The writer wishes to express 
his indebtedness to Prof. Livingston for much assistance in carrying out the 
study and for helpful criticism in the preparation of manuscript. The writer 
also wishes to thank Dr. H. E. Pulling for valuable suggestions made during 
the course of the study. 

The stations employed were Oakland, Chewsville, Monrovia, College Park, 
Baltimore, Darlington, Coleman, Easton, and Princess Anne. One station, 
Oakland, is in the Allegheny plateau. Four stations are in the Piedmont 
plateau; one of these (Chewsville) being in the Hagerstown valley, two (Darl- 
ington and Monrovia) in the hilly country north and west of Baltimore, and 
one (Baltimore) at the lower edge of the plateau near Chesapeake bay. Four 
stations, College Park, Coleman, Easton and Princess Anne, are in the coastal 



6 McLean, F. T. A Preliminary study of climatic conditions in Maryland, as related to plant growth. 
Physiol. Res. 2: 129-208. 1917. 



Climatic Conditions of Maryland 345 

plain. Coleman, Easton, and Princess Anne, are east of Chesapeake bay, 
while College Park is west of it and much farther inland, near the line of de- 
marcation between the coastal plain and the Piedmont plateau. All of the 
stations except Oakland are at comparatively low elevations — less than 310 
meters (1000 feet) above sea-level. Oakland has an elevation of 775 meters 
(2500 feet). The geographical distribution (see fig. 1) of these stations is 
such as to insure considerable differences in climatic conditions. 

At each of the nine places referred to, a series of cultures was grown in 
the open with no covering other than a screen of large-meshed wire netting. 
These have been termed the exposed stations. Also, at Oakland, Baltimore, 
and Easton series of cultures were grown under glazed cold-frame sash sup- 
ported horizontally 1 meter (3.3 feet) above the surface of the soil. These 
have been termed the covered stations. The plants were freely exposed at 
the sides, the sash being supported merely by corner posts. They were 
located within a very few meters of the enclosures containing the plants of 
the exposed stations and were subjected to the same climatic conditions as 
the exposed plants, except in so far as these conditions were modified by the 
glass covers. Furthermore, a series of cultures was exposed in the woods 
hear the Laboratory of Plant Physiology of the Johns Hopkins University at 
Baltimore. This has been termed the Baltimore forest station. As in the 
case of the exposed stations, these forest cultures were covered by a pro- 
tective screen of wire netting. They were, of course, subjected to a complex 
of climatic conditions quite different from those of the exposed and covered 
stations at Baltimore. The forest station was distant about 150 meters (490 
feet) from the exposed and covered stations, at Baltimore. There are thus 
plant data. available from 13 series of cultures in all, each series having been 
exposed to a different set of environmental conditions throughout the season. 

The cultures were so planned that the plants might be considered as stand- 
ard plants for the comparative measurement of climatic conditions in accord- 
ance with a suggestion made by Livingston and McLean. 6 Since the prob- 
lem of expressing plant growth in terms of the intrumental measurements of 
the climatic conditions that control it is rendered exceedingly complex by the 
number of these conditions and by their continual variation, as well as by 
the changing internal conditions of the plant itself, a detailed analysis of 
the control of plant growth is very difficult. Livingston and McLean sug- 
gest that the rate of growth of any plant is itself an expression of the summa- 
tion of all the effects of the external conditions acting during the growing 
period, and that a standard plant may be employed as an automatically weight- 
ing, integrating, and recording instrument for the comparative measurement 
of environmental conditions as these are effective in growth control. Thus 
several environments may be measured and compared in terms of their sev- 



6 Livingston, B. E., and McLean, F. T. A living climate-logical instrument. Science 43: 362. 1916, 




346 



Climatic Conditions of Maryland 347 

eral capacities for producing growth in the standard plant. This method of 
measuring the environmental complex in terms of plant growth can be applied, 
of course, only when it may be assumed that all the standard plants are alike 
at the beginnings of the several periods of exposure. In this study the re- 
quirement just stated was fulfilled by employing the seed as the starting point 
for the plants of the various cultures. It was apparent that if the cultures 
were always started from the seed, the plants might be considered as more 
nearly alike at the beginning of the several culture periods than would have 
been the case if an attempt had been made to obtain like plants in any other 
phase of their development. 

Growth rates were measured and compared in terms of size and weight of 
the plants. Each culture consisted of six plants grown for a period of four 
weeks from, the seed. Cultures were started approximately every two weeks 
during the growing season, at each of the stations employed, and growth 
measurements were made after about two weeks and again after about a 
month. The plants were harvested at the end of the longer period. 

As in other problems in which a number of conditions enter into the control 
of a process, the relations between conditions and process rate are more easily 
detected the smaller is the number of conditions involved, and conditions 
may be temporarily left out of consideration if they are the same in several 
experiments. Just as the internal conditions of the standard plant at the 
start of the experiment are left out of the argument by the simple device of 
having them all alike at the beginning of the exposure period (the instrument 
being thus set at the zero of its scale, in the words of Livingston and McLean), 
so selected conditions of the surroundings may be left out of consideration by 
having them alike throughout all of the periods. According to this principle, 
most of the environmental conditions that acted on the plants below the soil 
surface were kept nearly the same for all times and at all stations. Assuming 
that this artificial control of the subterranean environmental conditions kept 
them practically constant, the differences observed in the growth rates of 
the standard plants were taken to be related almost entirely to the aerial 
conditions of the surroundings. These are the ones referred to by McLean 
as climatic, and this term will be used with the same meaning in the present 
paper. To accomplish this control of the subterranean conditions, the soil 
was always the same at the beginning of all cultures and its moisture content 
was generally kept approximately the same throughout all culture periods, 
by means of the Livingston auto-irrigator. This arrangement and its opera- 
tion have been described by McLean and will also receive some attention 
below. 

While several different plant species were employed throughout the experi- 
mental work, the present paper deals only with the data obtained from soy- 
bean. A variety of this plant called "Peking," was used. The seed was of 
pure strain obtained from the 1913 crop of the Maryland Agricultural Experi- 



348 F. Merrill Hildebrandt 

ment Station. All the seeds were first treated with carbon bisulphide vapor 
for one week, to destroy insects, after which they were placed in paraffined 
paper cylinders with tight-fitting covers and stored until ready for use. 

A rather light soil, of the type classified as Norfolk Sand by Bonsteel, 7 
obtained from an untilled field near College Park, Maryland, was used in 
all of the plant cultures. This soil was chosen for three reasons: first, its 
water-holding power is comparatively low, so there was little danger of long- 
continued excessive water-logging after a rain ; second, it was not a very fertile 
soil and yet was capable of giving good growth under favorable climatic con- 
ditions; and finally, it was conveniently obtained. The method of obtaining 
it was to remove the top soil to a depth of 15 cm. from a small area of the 
field. This top soil was then thoroughly mixed, sifted, and placed in cloth 
sacks for shipment to the various stations where it was stored in air-dry 
condition in covered, water-tight, galvanized iron cans until needed for use 
in the cultures. The soil containers for the cultures were ordinary "6 inch" 
porous clay flower-pots, in form like the frustrum of a cone, being smaller at 
the bottom, and of a cubic capacity of approximately 1980 cc. 

In order to secure uniform soil conditions in the various cultures, it was 
necessary, not only that the soil should be of the same character in all of 
them, but also that it should be brought into the same physical condition for 
the beginning of all cultures. Furthermore, it was desirable that this physical 
condition be such that it might be retained with as little change as possible 
during the growth periods of the plant. To put the soil into a state of aggre- 
gation to be least altered by varying weather conditions (especially heavy 
rains which pack it more or less) it was saturated with water immediately 
after being put into pots. This was accomplished by plunging the filled 
pots into a bucket of water and allowing them to remain submerged until 
air bubbles ceased to rise. The pots were then drained and a two-week inter- 
val was allowed to intervene before the seeds were planted. 

The soil moisture in the cultures was maintained always above a certain 
minimum by means of auto-irrigators. 8 This device, as here used, consisted 
of two cylindrical porous clay cups (of the regular form supplied by the "Plant 
World") connected with each other and with a water reservoir by glass tubes 
each having the form of an inverted J. The cups were placed vertically in 
the pot, their rubber-stoppered tops level with the soil surface, and were so 
arranged as to supply water to the soil against a pressure of 35 cm., or some- 
what more, of a water column. The moisture content of the soil was thus 
maintained so that it was never less than about 10 or 11 per cent on the basis 



'Bonsteel, J. A. The Soils of Prince George's County [Maryland]. Maryland Geological Survey: Balti- 
more, 1911. 

8 Livingston, B. E. A method for controlling plant rrioisture. Plant World 11: 39-40. 1908. Livingston, 
B. E. Auto-irrigation of pots of soil for experimental cultures. Carnegie Inst. Washington Year Book 14: 76. 
1916. 



Climatic Conditions of Maryland 349 

of dry weight. With this water content this particular soil was rather too 
wet than too dry for the best growth of the plants. 

After preparing the pots and arranging the watering devices the pots were 
allowed to remain fallow for about two weeks before planting, as has been 
mentioned. Thus the soil was fully drained after the preliminary saturation 
and had settled into a condition somewhat approaching that of structure 
equilibrium, before the seeds were planted. The seeds were placed 2.5 cm. 
deep, six seeds in each pot. Care was taken to space them uniformly from 
each other, from the auto-irrigator cups and from the sides of the pots, so 
that all should have, as nearly as possible under the general conditions of the 
experiments, the same soil moisture conditions. When the plants were 
removed from a pot (about six weeks after that pot had been filled) the soil 
was discarded and fresh soil from the stored supply was used in refilling for the 
next following culture. 

THE OBSERVATIONAL DATA AND THE AVERAGES DERIVED 

FROM THEM 

The Plant Measurements 

The first plant measurements were made after approximately two weeks of 
growth from the seed. At that time the length of each leaflet, from the tip 
to the junction of blade and petiole, was determined, as was also the greatest 
width of each leaflet, measured at right angles to the long axis. The height 
of each plant was also measured, from the soil surface to the base of the termi- 
nal bud. At the end of approximately four weeks of growth the height 
measurement was repeated, after which the plants were cut off at the soil 
surface and photographic prints were prepared of the fresh leaves. By 
means of these leaf-prints the leaf area (one side) was afterwards determined 
planimetrically, for each leaf and for each plant. Finally, the dry weight of 
tops for each culture was determined. All linear measurements were made 
to the nearest millimeter, areal measurement to the nearest 0.1 sq. cm., and 
weight measurements to the nearest 0.01 g. 

For convenience, these five kinds of observational data on growth rates are 
summarized below. 

After about 2 weeks of growth: — 

(1) Stem, height, millimeters. 

(2) Length and breadth of all leaflets, millimeters. 
After about 4 weeks of groivth: — 

(1) Stem height, millimeters. 

(2) Leaf area (one side), square millimeters. 

(3) Dry weight (of tops), milligrams. 



350 F. Merrill Hildeerandt 

Of course the growth data obtained by actual observation require averag- 
ing in some way so as to represent the various periods throughout the season. 
The ordinary method of averaging was applied in all cases, but the observa- 
tional data for leaflet dimensions required special preb'minary treatment. 
The two dimensions derived from the measurements were multiplied together 
for each leaflet, giving the leaflet-product and these products were summed to 
give the total leaf-product of each plant. As McLean pointed out, the mean 
daily rate of increase in total leaf-product for a period of 4 weeks is very nearly 
proportional to the corresponding rate of increase in actual leaf area, and it 
seems safe to suppose, as McLean did, that the 2-week leaf-product values 
may be regarded as indices of increase in the area of the leaves. These 
product values are therefore used, for the 2-week periods of this study, as 
indicators of the rates of leaf expansion. Leaf areas were of course not deter- 
mined for the 2-week periods, although they are available for the 4-week 
periods. 

There were six plants started in each culture, but in many cases the num- 
ber of plants from which records were actually taken was less than six (it 
was never less than three and was usually four or five in such cases), on account 
of noticeable injury due to other conditions than the ones here studied, such 
as insect attack, etc. All plant data have therefore been reduced to averages 
per plant. Also, in many cases the length of the period differed slightly from 
14 days for the 2-week periods, and from 28 days for the 4-week periods, and 
the averages per plant have consequently been expressed as mean daily values 
for the respective periods. This method renders the plant measurements for 
the different periods more strictly comparable. It should be noted, however, 
that the growing periods were 14 and 28 days long in the majority of cases, 
and that variations in the length of the culture period were slight. Consid- 
ering the 2-week and 4-week plant values as measures of the results of plant 
processes acting through these periods, the mean daily values represent mean 
daily increments or process rates, and they will be termed "daily increments," 
for their respective periods, in the discussion that follows. Thus, for a 
plant 10 cm. high at the end of a 13-day period, 10/13 (or 0.77) cm. is regarded 
as the mean daily increment of increase in height, for that period. Letting 
the word growth represent the particular process to which the given kind of 
measurement refers (as increase in height, increase in leaf area, etc.), these 
may be spoken as growth increments. 

The mean daily growth increments, and also the mean daily climatic 
values, for the respective periods have been expressed in terms of the corre- 
sponding average of all the periods considered, for all exposed stations. This 
procedure renders all the values directly comparable. To obtain this unit 
for any land of value, all of the corresponding values (as all 2-week daily 
mean increments in height, for example, for all exposed stations) were summed 
and the sum was divided by the number of values summed. Then each indi- 



Climatic Conditions of Maryland 351 

victual value was divided by the unit thus obtained. The data were expressed 
as these ratio values, which will be termed relative values in the following 
discussion. To avoid decimals, these relative values have all been multiplied 
by 100, and are thus given in the tables and in the text of this paper. The 
absolute magnitude of the unit used for expressing each kind of value is of 
course not important; it is essential only that all comparative values be ex- 
pressed in terms of the same unit. The unit here employed represents in 
every case simply the average of all similar quantities that are used in the 
present study. If another station had been employed, or if the season had 
been longer or shorter at any station, the values of these comparative units 
would have been different. The magnitudes of these units thus depend to 
some extent upon the climatic conditions encountered at the various stations 
in the summer of 1914, to some extent upon the number and location of the 
stations, to some extent upon the nature of the soil used in this investigation, 
and to some extent upon the physiological nature of the soy-bean plant. 
The actual values of the various growth units are given below. 
For first 2 weeks of growth from seed 

Average daily increment of stem height per plant, 3.56 mm. 

Average daily increment of leaf -product per plant, 112.0 sq. mm. 
For first 4 weeks of growth from seed 

Average daily increment of stem height per plant, 3.20 mm. 

Average daily increment of leaf area per plant, 122.0 sq. mm. 

Average daily increment of dry weight (of tops) per plant, 6.29 g. 
The use of the relative values described above simplifies the plotting of 
the graphs upon which interpretation of such a study as this so largely de- 
pends. It also renders possible a direct comparison between the values for 
any two cultures irrespective of their dates or stations. Furthermore, it is 
possible to tell from the magnitude of the relative value for any culture the 
extent to which the plant, or climatic, measurement under consideration 
departs from the mean of that measurement for all the cultures of the study. 
To obtain the original or absolute plant,- or climatic, value from a given 
relative value, it is necessary only to reverse the arithmetical procedure by 
which the relative value was derived. For example, suppose it is desired to 
get the actual mean daily rate of increase in leaf-area per plant for the four- 
week period ending September 2, and for the station at Coleman. The relative 
value given in the table is 108. The first operation is to divide by 100, which 
gives 1.08 as the true relative value. The average daily increment in leaf 
area per plant, for the period and station in question, was therefore 1.08 times 
the value of the common unit employed for comparing the rates of increase 
in leaf area. Multiplying this unit value (122 sq. mm., as given above), by 
1.08 gives 132.0 sq. mm. as the actual mean daily increment required. To 
obtain the average total leaf area per plant at the end of the period in ques- 
tion, we multiply 132.0 sq. mm. by the number of clays in the period (28 in this 



352 F. Merrill Hildebrandt 

case) and get 3698 sq. mm. Since there were 5 plants measured in this cul- 
ture, the total leaf area for the entire culture at the end of the period is ob- 
tained by multiplying 3698 sq. mm. by 5, which gives 18490 sq. mm. or 184.9 
sq. cm., which is the actual areal value determined from the prints of these 
leaves. All of the original absolute values may be obtained from the rela- 
tive ones in a similar manner. It is of course evident from the above de- 
scription of the manner in which the relative values have been derived that 
they are proportional to the corresponding absolute values. In all subsequent 
discussion, when plant (and also climatic) values are referred to, it will be 
understood that these are the relative values rather than the absolute ones. 
As in the case of the observational data, the five kinds of derived growth 
values used in this study are listed below, for convenience. 
After about 2 weeks pf growth from seed 

(1) Relative mean daily increment of stem height per plant. 

(2) Relative mean daily increment of total leaf-product per plant. 
After about 4- weeks of growth from seed 

(1) Relative mean daily increment of stem height per plant. 

(2) Relative mean daily increment of total leaf area per plant. 

(3) Relative mean daily increment of dry weight (tops) per plant. 

The Climatic Measurements 

The weather observations taken by the cooperative observers at the sev- 
eral stations here employed consisted of daily readings of maximum and 
minimum thermometers, daily ocular observations of cloudiness, daily meas- 
urements of rainfall, and general notes as to storms, wind, etc. The records 
of the Baltimore office of the Weather Bureau were used for the Baltimore 
stations. In addition to these records of the weather observers, evaporation 
was measured by means of Livingston standardized cylindrical porous cups 
with non-rain absorbing mountings. 9 Of the five sets of climatic observa- 
tions mentioned above, only three will be considered in this paper; namely 
those of temperature, light, and evaporation. As was pointed out by McLean, 
rainfall showed little or no relation to the growth of these plants, as was in- 
deed to be expected, since the soil moisture of the cultures was always kept 
sufficiently high (by the auto-irrigators) for the needs of the plants. Also, 
the miscellaneous climatological observations reported by the weather observ- 
ers will not be considered in this paper, since none of them have been found 
to bear any discoverable relation to the growth rates of these plants. The 



'Livingston, B. E. A rain-correcting atmometer for ecological instrumentation. Plant World 13: 79-82. 
1910. — Livingston, B. E., and Shive, J. W. The non-absorbing atmometer. Carnegie Inst. Washington Year 
Book 13: 93-94. 1915. — Shive, J. W. An improved non-absorbing porous cup atmometer. Plant World 18: 7-10. 
1915. — Johnston, E. S. A simple non-absorbing atmometer mounting. Plant World 21: 257-260. 1918. — Liv- 
ingston, B. E., and Thone, Frank. A simplified non-absorbing mounting for porous porcelain atmometers. 
Science 52: 85-87. 1920. 



Climatic Conditions of Makyland 353 

observational data were obtained daily throughout the entire season. For 
convenience, these three kinds of observational data on climatic conditions 
are listed below. 

(1) Temperature. 

Daily maximum and minimum air temperature (shade), de- 
grees, Fahernheit. 

(2) Light. 

Daily light condition, whether clear, partly cloudy, or cloudy. 

(3) Evaporation. 

Daily evaporation from standardized cylindrical porous-cup 

atmometer, cubic centimeters. 

The climatic data, like the plant data, require special treatment before 

they can be used in such a study as this. How the mean values for the 

various periods were secured from the observational data will be described, 

for each of the three kinds of climatic measurements, in the following sections. 

TEMPERATURE 

It is clear that the readings of a thermometer do not express the effective- 
ness of various degrees of temperature to accelerate or retard plant growth, 
and it therefore becomes desirable to replace the actual thermometer readings 
by a series of weighted values, more or less directly proportional to the tem- 
perature effect upon the growth of plants. Owing to lack of information of 
a quantitative nature as to the relation between plant growth and environ- 
mental temperature, this can be accomplished only in a tentative and approxi- 
mate way at the present time. 

The observational data for temperature were all obtained from maximum 
and minimum thermometers read daily at sunset, these data being taken 
from the published monthly reports of the U. S. Weather Bureau. 10 The 
mean temperature for each day was determined by averaging the maximum 
and minimum for that day. McLean has discussed some of the ways in 
which daily maximum and minimum temperature data may be treated in 
order to obtain weighted values that may tentatively represent temperature 
effects upon plant growth rates. He emphasizes the fact that temperature 
values, as shown by a thermometer, do not show a linear proportionality to 
plant growth. If thermometer readings might be taken as expressing, even 
in an approximate way, the effectiveness of temperature to produce plant 
growth, such a relation could only be true up to the optimum temperature, 
since beyond tins point increased temperature results in decreased growth. It 
would therefore be desirable to replace each thermometer reading by an 
index representing the effectiveness of that particular temperature for plant 



10 Fassig, O. L. Climatologieal data, Maryland and Delaware Section. May to November, inclusive. U. S. 
Weather Bureau. 1914. 



354 F. Merrill Hildebrandt 

growth. Three ways of doing this, all of which have been considered by 
McLean, may receive brief mention here. 

(1) One way of expressing temperature, which has been used in ecological 
studies, has been called the remainder-summation method. This is based on 
the supposition that the growth activities of most plants stop when the 
temperature falls below 40° F. 11 Above this temperature, growth increases 
with increased temperature, to an optimum. For convenience, the growth 
rate for 40°F. may be considered as unity: then it should be 2 for 41°, 5 for 
44°, 20 for 59°, etc. If we subtract 39° from any given temperature, the 
remainder will represent, according to this method, the efficiency of the given 
temperature for producing growth. A total efficiency value for any period 
of time, such as the 4-week growth periods of these studies, may be obtained 
by subtracting 39° from each daily mean temperature and summing the 
remainders for the period, thus obtaining the remainder summation. 

(2) Another method of weighting temperature values for the purpose 
before us, and one that apparently has a somewhat more rational basis, was 
suggested by Livingston and Livingston. 12 They proposed a series of tem- 
perature-efficiency indices based on the van't Hoff-Arrhenius law, which 
states that the velocity of many chemical reactions approximately doubles 
with a rise in the temperature of 10°C. (18° F.). If it is assumed that the 
growth rate for plants follows this law above 40° F., at which temperature 
the rate is taken to be unity, a series of values representing temperature 
efficiencies for higher temperatures may be derived. When this scheme is 
used, the efficiency value for any temperature is represented by the value of 
the index that corresponds to the temperature value itself. Assuming the 
growth rate to be unity for a temperature of 40° F., it should be 1.21 for a 
temperature of 45°, 2.0 for 58°, etc. These indices are called by Livingston 
and Livingston "exponential indices," and they have published a table of 
these values. 

Since most of the temperatures with which we have to deal are below the 
optimum for plant growth, since temperature and the growth rate are related 
in an approximately linear manner between 40° F. and the optimum (about 
90° F.) , and since both the exponental and remainder series of index values 
increase in a nearly linear way throughout this range, both of the methods 
just considered give temperature efficiency numbers that are more or less 
approximately proportional to plant growth as it is influenced by tempera- 
tures between these limits. It is obvious, however, that neither of these 
methods can properly express efficiencies for temperatures above the optimum, 



11 The temperature data of this paper are all expressed in terms of temperature degreeson the Fahrenheit 
scale, simply because the observational data had this old-fashioned form. By retaining the Fahrenheit values 
much labor has been avoided, but it is not to be understood that the writer is really as conserva- 
tive as this feature of the paper might seem to suggest. 

12 Livingston, B. E., and Grace J. Livingston. Temperature coefficients in plant geography and clima- 
tology. Bot. Gaz. 55: 349-375. 1913. 



Climatic Conditions of Maryland 355 

since they give numbers winch continue to increase with increasing tempera- 
ture, while growth increases with increasing temperature up to the optimum 
and then decreases with higher temperature. Also, both these methods give 
results that are approximately proportional to each other for ordinary sum- 
mer temperatures. This fact has been noted by Livingston and Livingston 
and again by Stevens, 13 and it is also obvious from the climatic data given by 
McLean. But it must be remembered that these methods cannot be satis- 
factory excepting when the temperatures dealt with he mainly between 40° 
and 90° F. 

When the exponential indices are employed each daily mean temperature 
for any period is replaced by its index and the series of indices thus obtained 
is summed for the period, giving the exponential summation. 

(3) The third method of expressing temperature values as they affect plant 
growth has been more recently suggested by Livingston. 14 It is based on the 
results of Lehenbauer's experiments with maize seedlings. From. Lehen- 
bauer's data, Livingston derived a series of coefficients giving the efficiencies 
of various temperatures in terms of the growth of this plant. He has called 
these "physiological temperature indices." The growth rates upon which 
the index values were based are those shown by Lehenbauer's seedlings when 
exposed for 12 hours to various maintained temperatures, the other condi- 
tions of the experiment being approximately the same for all tests. Living- 
ston suggests that the coefficients thus derived from the growth of maize 
under controlled conditions, with different maintained temperatures, may 
possibly express some approach toward a general relation between plant 
growth and temperature and may thus be applicable to plants growing under 
other conditions. The graph of these physiological indices exhibits the same 
direction of slope between a low temperature and the optimum as do the graphs 
of temperature efficiencies derived by the other two methods, but for this por- 
tion of the temperature range the slope of the graph of physiological indices is 
generally steeper than that of the graph of remainder indices, the latter graph 
itself having a much steeper slope than that of the exponential indices. This 
is shown by Livingston and also by Stevens, in the papers cited above. 
Since they are derived from the actual growth rates of a plant, the physiological 
temperature indices appear to have a more rational basis than either the 
remainder or the exponential indices. For this reason, and for others that 
will appear below, physiological indices have been used in this study for ex- 
pressing the temperature values as they are to be compared with the plant 
growth-rates. 



13 Stevens, Neil E. Influence of temperature on the growth of Endothia parasitica. Amer. Jour. Bot. 
4: 112-118. 1917. — Idem. Influence of certain climatic factors on the development of Endothia parasitica. 
Ibid. 4: 1-33. 1917. 

11 Livingston, B. E. Physiological temperature indices for the study of plant growth in relation to cli- 
matic conditions. Physiol. Res. 1: 399-420. 1916. — Lehenbauer, P. A. Growth of maize seedlings in relation 
to temperature. Physiol. Res. 1: 247-288. 1914. 



356 F. Merrill Hildebrandt 

In using the physiological temperature indices for the purposes of this 
study, the procedure has been as follows. Each daily mean temperature for 
any period has been replaced by its corresponding index (taken from Living- 
ston's table, — 1916) and then all the daily index values have been summed 
to give the physiological summation for the period. This summation value 
is finally divided by the number of days in the given period. 

Two other series of temperature values are presented in the tables of this 
paper, but neither has been found to be as satisfactory for expressing this 
climatic condition as are the physiological-summation indices. These are 
(a) the average daily mean temperature for each culture period (in degrees, 
Fahrenheit) and (b) the remainder-summation index for each period. 

As has been stated, the physiological-summation indices for all periods 
have been represented as daily means, and these have been stated always as 
relative values, in terms of the general average for all periods and stations. 
The average value used as unity in expressing the relative index values for 
temperature is 56.39. 

The following considerations may be added to show the reason for using 
the physiological-summation indices in this study, secured as above described, 
rather than the remainder-summation or exponential-summation indices. 
It will be necessary to anticipate somewhat the discussion that is to follow 
this section. The three climatic conditions (temperature, evaporation and 
light) each show a definite seasonal march for each of the places employed in 
this study. The temperature rises from low values in the spring to a mid- 
summer maximum, which is followed by a subsequent fall to low autumnal 
values. On the other hand, the values representing light and evaporation 
both decrease, in general, throughout the season. If, now, a generalized 
curve representing the growth of the plants be drawn, employing average 
values to represent all the stations together, and plotting them as ordinates 
with the dates of the middles of the periods as abscissas, such a growth curve 
follows the seasonal march of temperature and shows only secondary varia- 
tions as related to the other two climatic indices. The growth of the plants 
is thus apparently determined mainly by temperature. Obviously, also, the 
seasonal march of the temperature values must show the same general form 
of curve no matter what scheme is used in expressing temperature efficiency. 
In view of these facts, and in consideration of the general comparative pur- 
pose of the present study, a method should be used, for expressing tempera- 
ture efficiency, that gives a seasonal march of the efficiency values in accord 
with the corresponding march of generalized plant growth. Of the three 
methods mentioned, the physiological-summation index fulfills tins require- 
ment best, and this has accordingly been selected for use throughout the 
entire study, as has been said. 

An examination of the plant and climatic graphs (to be considered later) 
shows that the plant values for most of the stations rise above the temperature 



Climatic Conditions of Maryland 357 

efficiency values in the middle of the season, and fall below them, at its end. 
Tins is probably due in part to the effect of light and evaporation but it may 
also be related to an inadequacy of the temperature efficiency values to repre- 
sent the actual effect of temperature on the growth of these plants. It 
appears to be at least suggested that the actual temperature efficiency values 
for these so3 r -bean plants increase more rapidly with increase in the tempera- 
ture itself, for the range here encountered (between 40° and 85° F.), than to 
the physiological index values derived from Lehenbauer's study of maize 
seedlings. This whole question deserves much more experimental study. 
It is a surprising fact that we have available only a single thoroughgoing in- 
vestigation (Lehenbauer's) of the relation of temperature to the growth of 
higher plants, in spite of the fact that the primary importance of the tem- 
perature control of growth is obvious to every observer and has long been 
qualitatively appreciated. A comparison, for any of the stations employed, 
of the range of growth values for the plants with the remainder-summation 
values for temperature (which are practically equivalent to the exponential- 
summation values in this study) and with the physiological-summation in- 
dices will furnish evidence for the verification of these statements. The 
graphs of the physiological-summation indices of temperature efficiency show 
much steeper slopes than do the corresponding graphs derived from the other 
two kinds of temperature indices mentioned above, however, so that the physi- 
ological indices are evidently more suitable to represent temperature effici- 
encies than are either of the other two kinds of indices. 

LIGHT 

The only records of light conditions that were available for all of the sta- 
tions of this study were the daily ocular estimates of cloudiness obtained by 
the weather observers, and these alone were used. To make use of these 
estimates it was, of course, first necessary to bring the dairy percentages of 
clear "sky together for each culture period, so as to derive for each period a 
single value that might be taken to represent the intensity of the light condi- 
tion for that period. The method employed to accomplish this is presented 
below. 15 

The total heat equivalent of the actual sunshine for any given period at a 
given station is primarily a function of three terms: (1) the maximum possible 
number of hours of sunshine (determined by latitude and season); (2) the 
mean daily intensity of full sunshine for the period and station, which may 
be expressed in terms of units of heat received per unit of a horizontal sur- 
face ; (3) the condition of the sky, whether overcast, partly overcast or clear. 
The daily values for the first two of these terms vary in a regular manner 



15 The presentation of this method is here practically the same as that previously published. See: Hilde- 
brandt, F. M. A method for approximating sunshine intensity from ocular observations of cloudiness. Johns 
Hopkins Univ. Circ, March, 1917. 



358 F. Merrill Hildebrandt 

throughout the year for any given place, and the ones for the third term are 
roughly stated in the observer's records, as just mentioned. 

The first two terms are combined in the ordinates of the graph given by 
Kimball 16 for the maximum possible total radiation received per day at 
Mount Weather, Virginia. Since this station is at about the same latitude 
as the stations here dealt with, the ordinates from Kimball's graph may be 
taken as approximate measures of the total maximum possible light intensi- 
ties for the corresponding dates for all of the Maryland stations. These 
values represent the total amount of heat received from the sun and sky on 
clear days at Mount Weather, in gram-calories per square centimer of a hori- 
zontally exposed surface. The method of using this graph along with the 
weather observer's reports, for estimating sunshine intensity for any station 
and period, will be best shown by an example. Suppose it is desired to esti- 
mate the average daily sunshine intensity for some station in the general 
region of Mount Weather, for the first week of August. The average ordi- 
nate value for this week is first obtained from Kimball's graph. For periods 
as short as a week or two this may be done by averaging the values for the 
first and last days of the period, since the curve may be taken as a straight 
line for such short intervals. The ordinate values for August 1 and August 

7 are ■ and , and their average is . From the report of 

the weather observer at the place in question, the number of clear, partly 
cloudy, and cloudy days is next determined for the days August 1 to August 
7, inclusive, and some arbitrary weighting is given to each kind of day. This 
was done in the present instance by regarding days reported "clear" as days 
of full sunshine, those reported "partly cloudy" as half days of sunshine, 
and those reported "cloudy" as without any sunshine. The same scheme 
of weighting must of course be adhered to in all the estimates used for com- 
parative purposes in any discussion. Suppose there were 2 clear days, 3 half- 
cloudy days and 2 cloudy days. By summing these values as 2, 1.5 and 0, 
we obtain 3.5, representing the equivalent number of wholly clear days for 
the period considered. Now, 3.5 is 0.5 of the total number of days in the 
week period, and the latter value may be termed "the coefficient of clear 
weather." By multiplying the average daily intensity value for clear days, 

, (obtained by the use of Kimball's graph) by this coefficient of clear 

weather (0.5) we obtain g.-cal. as a rough approximation of the 

average daily sunshine index for the week. 

While it is certain that solar radiation affects plants in other ways than 
through its heating effect, it is no less certain that by far the greater part of 
the energy of sunshine absorbed by plants is converted into heat (largely as 
latent heat of vaporization of water), and it seems probable that the other 
effects produced upon the plant may be more or less proportional to the total 



\' 16 Kimball, Herbert H. The total radiation received on a horizontal surface from the sun and sky at 
Mount Weather. Monthly Weather Rev. 42: 474-487. 1914. (See especially fig. 8, p. 484.) 



Climatic Conditions of Maryland 359 

energy equivalent of sunshine. This method of deriving sunshine indices is, 
however, to be taken only as a rough approximation. 

For each 2-week and for each 4-week period of this study an index of sun- 
shine intensity was secured in the manner just described, and each of these 
sunshine values was expressed in terms of the average of all values in the 
series. The relative values thus obtained are quite parallel with the other 
relative values already referred to. They are given in the tables of data and 
the actual values may be obtained from them in a manner like that described 
for the plant values. The general unit used in expressing these relative light 
values (the average daily sunshine intensity for all stations and for all periods) 
is 442 gram-calories per square centimeter of horizontal surface. 

evaporation 

The evaporating power of the air was measured as has been said, by means 
of standardized cylindrical porous-cup atmometers, located so as to have about 
the same exposure as the plant cultures. The instruments were read at inter- 
vals of about two weeks, the dates of reading being the same as those on which 
observations were made on the plants. After every reading each atmometer 
cup was removed and replaced by another that had just been standardized. 
The used cup was subsequently restandardized so as to detect any change in 
the coefficient of the cup consequent upon its exposure. When the restand- 
ardization showed a change in the coefficient, the mean of the original coef- 
ficient and the coefficient found upon restandardization was employed to 
reduce the reading to the Livingston cylindrical standard. The evaporation 
readings should therefore be directly comparable to other measurements 
related to the same standard, 

As has been pointed out by Livingston, 17 the porous-cup atmometer is 
somewhat similar to plant foliage in the way in which its evaporating surface 
is exposed to the surroundings. It may therefore be supposed that the 
transpiration from the plants for any period should be approximately propor- 
tional to the evaporation from the atmometer, except in so far as the trans- 
piration rates may be influenced by conditions within the plant. The work 
of Briggs and Shantz indicates that evaporation from small open pans or 
porous cups is influenced by the same external conditions, and in about 
the same way, as is plant transpiration, if the comparison is made for periods 
of a day or more. Of course the two rates do not vary proportionally within 
the day period, since the internal conditions of the plant exhibit a peculiar 
daily march, but with such details this study does not need to deal. It has 
been supposed, therefore, that the effectiveness of the external conditions 
to influence the transpiration rates of the plants of this study was approxi- 



17 Livingston, B. E. The relation of desert plants to soil moisture and to evaporation. Carnegie Inst. 
Wash. Pub. 50. 1906. 



360 F. Merrill Hildebrandt 

mately measured by the corresponding corrected evaporation rates from the 
atmometer. The atmometer readings have been reduced, in every case, to 
mean daily rates for the 2-week and 4-week periods, and these rates have been 
taken as indices of the evaporating power of the air as it affected transpiration 
from the plants. Finally, all atmometric values have been expressed rela- 
tively, in terms of the general average for all stations and for all periods, as 
in the case of the other data. The general average used as unity for these 
relative atmometric values is 16.2 cc. per day. 

The three derived climatic values in this study may be brought together 
here, for convenience. The list applies to the 2-week as well as to the 3-week 
series of data. 

(1) Temperature. Relative daily mean of the physiological-summation 
indices for the period. 

(2) Light. Relative daily mean of calculated light-intensity values for 
the period, gram-calories per square centimeter of horizontal surface. 

(3) Evaporation. Relative daily mean of atmometric indices for the per- 
iod, cubic centimeters of loss from the Livingston standard cylindrical porous 
cup. 

RESULTS AND DISCUSSION 

Introductory 

The discussion of the data obtained in this study will be devoted in part 
to descriptions of the growth changes observed in the plants at the various 
stations, and for the various periods at each station, and in part to corre- 
sponding descriptions of the climatic values. Owing to the complexity of 
the problem and to the number and variety of the data to be dealt with, it 
has been found necessary to depart frequently from a general logical order 
and to treat matters that seem to be of seondary importance at greater 
length than might appear necessary from a more restricted point of view. 
Such physiological interpretations as are attempted in the course of the 
presentation of the data are of interest partly for their own sake, but more 
particularly because of the bearing they may have on the general problem of 
the use of standard plants for the comparative integration of effective climatic 
complexes. The work here reported was planned primarily to make a first 
trial in the use of standard plants in this way. The discussion of the data 
will be presented more in the form of a running narrative, with digressions at 
many points, than is ideally desirable, but the newness of this land of study 
and the fact that the fundamental principles and even the terms to be em- 
ployed have yet to be developed, make anything approaching a true logical 
sequence quite impossible now. 

The various kinds of data to be considered will be brought forward in groups 
corresponding to their sources. The 2-week plant data and the 2-week cli- 



Climatic Conditions of Maryland 361 

matic data for the stations in the open will first be presented, followed by a 
presentation of the 4-week plant and climatic data for these stations. Sub- 
sequently, a special discussion of the data for the covered stations and a 
similar treatment of the data for the forest station at Baltimore will be 
given. 

The relative numbers, or indices, derived as described above, are given in 
tables I to VIII, together with the dates of the first and last days of each 
culture period and other information, including the length of each period, 
the number of plants in each culture, etc. Also, a set of figures is presented 
showing graphically certain parts of the information given in the tables. 
Tables I to VIII give twenty-six sets of data. Nine of these sets (tables I- 
III) contain the plant and climatic measurements for the 2-week culture 
periods for the exposed stations, nine others (tables IV-VI) give the data for 
the four-week culture periods for the exposed stations, six others (tables VII- 
VIII) give the data for the 2- and 4-week culture periods for the covered 
stations at Oakland, Baltimore and Easton, and the two remaining (table 
VIII) give the data for the 2- and 4-week culture periods for the Baltimore 
forest station. 

In each of the eight tables just mentioned, the first line gives the name 
of the place referred to, the kind of culture period (whether 2- or 4-week), 
the character of the exposure of the plants (whether the station is exposed, 
covered, or forest, and the dates of the beginning and end of each culture 
period. The second fine of each table gives the serial culture numbers. These 
numbers being assigned to the various cultures for convenience of reference. 
When several kinds of stations occur at one place, cultures of the same num- 
ber cover approximately the same time period. For instance, for Baltimore 
there is an exposed station, a covered station, and a forest station, and there 
is a 2-week culture "8" for each of these three stations, the dates for each 
of these being August 20 and September 3. In some cases, culture periods 
of the same number for the exposed and covered stations show a difference 
of a day in the lengths of their respective periods owing to the fact that it 
was impossible to take measurements on both the exposed and covered 
plants on the same day. The third fine of each table gives the length of each 
culture period, in days. The fourth line gives the number of plants actually 
used in obtaining the plant measurements. A dash appearing in place of a 
relative value indicates that the data necessary for calculating this value 
are lacking. An asterisk placed opposite a. climatic index value shows that 
this particular value was not plotted in the graphs (to be described later). 
(Points are omitted from the climatic graphs in the case of most cultures 
where no plant data are available for comparison with the climatic values.) 
The remainder of the table presents the relative plant and climatic values, 
the derivation of which has already been made clear. The last column of 
each of the tables gives the seasonal averages for the station considered. 



362 



F. Merrill Hildebrandt 



TABLE I 
Two-week data for exposed stations, Oakland, Chewsville and Monrovia 



OAKLAMD 

e-wcek periods. 
EX.P05EO 3TATIOM. 


Z3 


JUNE 

5 

JUME 
J9 


JUME 

19 

JULV 


JULY 
3 

JULV 
l& 


JULV 
l& 

JULV 
31 


3" 

AUG 

If 


AUG 
AUG 

2r 


AUG SEPT 
27 12. 

SEPT SE:PT 
12 25 








AV. 


Culture number 


l 


£ 


3 


4 


3 


6 


7 


o 


9 










Lenqtb of qrowinq period, daqs. 


13 


1** 


14 


13 


15 


14 


i 3 


l& 


'3 










Number of plants. 


1 


5" 


5 


& 


.S 


6 


fc> 


€» 


4 










ffemoinder summation Index.- 


334 


354 


•lot 


364 


431 


3to9 


343 


370 


262 








3fol 


Average daily relative pbsisiolocjital 
temperature, inden 


C?ty 


t>6 


S5 


78 


©7 


67 


64 


66 


43 








S9 


Avcraqc dai ly mean t^mperature.deq T 


65 


64 


<=>e 


far 


6e 


e»5 


t,G 


62. 


foi 








65 


Averaqe doilu relative evaporation 
index. , 


153 


ijq 


9S 


79 


104 


90 


7i 


5-7 


6,9 








96 


Averaqe duilu relative, sunshine- 
intensify 


122 


I2E 


i09 


103 


ne. 


102 


MO 


83 


ai 








>D5 


Averaqc daily relative, increment' in 
stem Ijcio, nt 


7G 


96 


124 


ai 


96 


qS 


ior 


S3 


42 








e& 


Averaqc dailu relative increment" in 
leaf- product 


- 


31 


a. 7 


74 


at? 


78 


82 


■46 


- 








7 7 



CHEWSVILLE 

2-woeK periods. 

EXPOSED STATION 


19 

June 


JLVCE 

JUNE 
16 


JUME 

JUME 

30 


June 
30 

JUlV 


JULY 
14 

JULV 

as 


JULY 
28 
AUG. 


AUG 
I I 

AUG, 
25 


AUl3 
Z5 

5C.PI 

a 


SEPT 
3 

SEPT 

22 


SEPI 

22 
OCT 

1 


OCT. 

7 
OCT. 

SO 




Av 


Culture number. 


1 


2 


3 


4 


3 


6 


7 


a 


9 


IO 


1 1 






Lenqtb o^- qrowinq period, du\js 


ii 


n 


H 


H 


,« 


(1 


H 


14 


i ^ 


13" 


13 






Number of plants 


6 


<b 


4 


& 


^ 


4 


3 


s 


& 


& 


6 






Remainder summation indcu. 


116 


•H3 


472 


1.53 


523 


450 


.517 


136 


302 


32© 


301 




4H.2 


Averaqe daily relative pbvjsioloqical 
tern pera+ure "index.. 


9? 


lOG 


iicj 


I iZ 


i19 


I iz. 


145 


I03 


4e 


51 


S3 




9S 


Averaqc daiU mean temperature, deq ft 


G.S 


7t 


71 


7l 


7G 


- 


TG 


TO 


ei 


Gi 


G2 




69 


Averaqc dail\j relative evaporation 
"mde*.. 


no 


l 15 


lOT 


H5 


ioe 


67 


io5 


ao 


7c» 


73 


61 




9l 


Averaqe daiKj relative sunshine 
intensity 


I2i 


129 


II& 


lOG 


iEl 


91 


qe 


67 


101 


73 


14 




9S 


Averaqc daily relative increment m 
stem laeiqhr. 


93 


93 


113 


lie 


I 26 


9a 


9S 


76 


ie 


37 


37 




S7 


Averaqc doilM relative, iiicrement" in 
lea} — product 


104 


1 lO 


I37 


qe 


I39 


1 27 


IOO 


52 


32 


20 


4 




Si 



MOMROVIA 

E-ujeck periods. 

EXPOSED iTATiOrt 


MAY 
IS 

jurtE 

i 


jurtE 
i 

JUNE 
15 


JUNE. 
15 

JUNE 
29 


jurte 
july 

)3 


JULY 

13 

JULY 
Z7 


JULY 
Zl 

AUG. 
IO 


AUG 
IO 

AUG. 
2*» 


AUG, 

Z<J 

5EPT 

7 


5EP7 

7 

SEiFI 

2J 


SEPT 
£1 

OCT. 

a 


OCT 

a 

OCT, 
19 




AV. 


Cullurc number. 


1 


£. 


3 


4 


5 


6 


7 


6 


9 


IO 


1 1 






Lenqrn of qrounnq period, daus. 


14 


M 


14 


14 


14 


14 


H 


14 


14 


17 


11 






number of" plants 


3 


•5 


G 


1 


fc> 


3 


6 


5 


& 


£> 


6 






^cma index- summation index. 


44 2- 


154 


17a 


4^f 


J 36 


4&3 


SI I 


433 


3lM 


36& 


£*>Z. 




431 


Aueraqe dail«j relative pbqsialoq ical 
temperotu'"e inde>. 


105 


MO 


121 


II2 


>5& 


H7 


114 


105 


59 


5"3 


59 




I04 


Averaqe dailv, mean temperature, deq F 


T\ 


7l 


73 


7 I 


77 


72 


7J 


70 


e>£ 


6S 


63 




no 


Averaqe daily relative evaporaTioo 

> ndei. 


- 


I4G 


!5£ 


93 


'17 


ieo 


MZ. 


oa 


96 


82 


76 




IO& 


Averaae dailv relative sunshine 
i nte n s 1 1 ^ 


I5Z. 


I22. 


101 


IO£. 


MO 


I05 


9S 


62 


t05 


G© 


J2 




IOO 


Pivera^e. JoiIlj relaTive increment" in 
stem, heiqlit 


73 


Si 


i£i 


96 


1 15" 


90 


107 


76 


20 


67 


25" 




T9 


Averaqe daily relative increment m 
leaf- product 


66 


90 


HO 


83 I3S 


I09 


lOI 


112. 


6 


3b 


4 




So 



Climatic Conditions of Maryland 



3G3 



TABLE II 
Two-week data for ex-posed stations, College, Baltimore and Darlinylon 



COLLEGE 

£-weeK per loch? 

EXPOSED £> rATlOM. 


MAI 

dune 

6 


JUNE 
JUNL 


JUMP 

JULY 


JU*-Y 
3 

JULY 

17 


JULY 

n 

JULY 
31 


July 
31 

AUG,, 
14 


AU<S 

AUG. 


acpr 

IO 


.ill 
IO 

31 1 I 


_.i .■ 

OCT 
IO 


OCT. 
IO 

ocr 

24 




AV. 


Culture number 


* 


^ 


-I 


•!> 


& 


7 


a 


9 


IO 


ii 


12 






Lenqth of- qrowmq period, da\(5. 


14 


13 


H 


l-l 


11 


14 


u 


14 


IS 


15 


14 






Number* of ple-vtts>. 


G 


•5 


-s 


fa 


4 


4 


5 


<b 


- 


& 


& 






Remainder summation index. 


5-40, 


•ton, 


526 


49 A 


szo 


'l"»4 


4 78 


441 


370 


.333- 


-327 




1iO 


Averaqe dail^ relative physiological 
temperarure index. 


108 


103 


139 


134 


lie 


133 


143 


US" 


e>7 


JO 


J3 




IQtJ 


Averaqe aailv, mean temperature, dcq. h" 


71 


70 


Tt. 


H 


76 


*4 


76 


7l 


o4 


<5l 


& a 




TO 


Avaraq»i daiKj relative evaporation 
index. 


1GO 


(54 


133 


9G 


143 


147 


i&S 


107 


73 


34 


ea 




■■*? 


Averaqe dail\j relative sunshine 
1 iTronoitv^. 


- 


- 


- 


- 


- 




- 


- 






- 






Avcroqc Jaiw rclaT'wc increment in 
stem heiqh-K 


qs 


132 


113 


MB 


124 


no 


90 


7(5 




48 


33 




=)S 


Averaqe dail, relative increment in 
leaf- produol: 


96 


IZ3 


SI 


152. 


203 


150 

1 


117 


G£ 


- 


13 


IO 




IOI 



BALTIMORE 

2-i-ueijk period ■=. 
EXPOSE D STATION. 


MAY 

14 

HAY 
29 


MAY 

29 

JUNE 
IO 


JUNE 
IO 

JUNE 
25 


JUNE 
25 

JULY 
9 


JULY 

9 
JULY 
£3 


JULY 
£3 

AUG. 
lb 


AUG. 
AUG. 


AU6 
20 

5EP7 

3 


5un 

3 
3EF7 


SEPT 

19 

OC7. 

J 


OCT. 

OCT. 
14 




Av. 


CliItutg number: 


1 


a 


"5 


4 


5 


& 


7 


S 


c t 


to 


II 






Lenqtb of-qrowmq period, duuj.3 


13 


12 


15 


14 


14 


14 


14 


■ 4 


ie> 


12 


13 






Number op plonts- 


s- 


- 


-S 


6 


& 


4 


5 


5 


& 


S 


.5 






'Rem cinder summah'on mde.x- 


45i 


370 


SOB 


4&9 


■946 


-SO 6 


543 


hOC 


593 


300 


37.9 




•4 48 


Aweraqe dailu relative phijs io logical 
temperature mden- 


9 1 


49 


IK3 


I25 


isa 


iv 


l&O 


139 


G2 


"1" 




IIS. 


Averaqe daiitj mean "temperature, decj f: 


G9 


70 


73 


73 


78 


73" 


78 


73 


G4 


&« 


q 




72 


Average da t l^ relative evaporation 

injcu. 


127 


IOZ 


US 


60 


92. 


112 


HO 


di 


»">9 


53 


G4 




93 


Averaqe da'i Nj relative sunshine 


137 


a? 


ioa 


go 


q 1 ) 


7a 


79 


Y4 


82 


89 


59 


— . 


36 


Averaqe am|\j relative increment m 
item heiqht. ■ 


9i 


- 


I4& 


152. 


183 


152 


152, 


149 


73 


5G 


96 


125 


Averaqe da,|-j relative increment in 
leaf-prodtAch 


IOG 


- 


125 


163 


194 


1 IO 


2.33 


ISO 


S-4 


as 


49 




119 



1 ■ r 

DARLIPSGTOrs 

2- uueeK pe riods. 

EXPOSED STATIOM 


MAY 

15 
MAY 

30 


MAY 
30 

JUNE 
13 


JUNE 
13 

JUNE, 
26 


JUME 

JULY 
IO 


JULY 

IO 

JULY 

24 


JULY 

24 

A UQ. 

7 


AU3 

7 
AUG. 

^I 


AUG 
21 

SEPT 
4 


3EP1 
A 

SSP7 
18 


13 

OCT 

2 


OCT 

OCT. 
16 




Av. 


Culture number. 


t 


2 


3 


■9. 


3 


& 


X 


3 


9 


IO 


1 1 






Lenqth of qrowmq period, do. ij.s. 


1-5 


14 


■ 3 


14 


14 


14 


14 


14 


M 


14 


14 






Number of plants. 


& 


■5 


e> 


4 


S 


S 


" 


•5 


« 


-5 


4 






Remainder sunn nation ii?dcx>. 


430 


432 


391 


432 


tJIZ 


456 


506 


471 


309 


3 IS 


336 




4IS 


Averaqe do>l^j relative ph^sioloqieal 
t"e m pe ratu. re index. 


37 


99 


94 


93 


149 


113 


146 


123 


4e 


S9 


54 




7l 


Averaqe daiUj meon temperature, cleq. f: 


Ge 


TO 


«9 


70 


76 


72. 


76. 


72 


61 


ce 


133. 




69 


Averaqe daiKj relqtive evaporation 
index. 


loo 


123 


90 


62 


74 


77 


79 


e>4 


82 


39 


39 




e 7 


Averaqe dailv, re\ative sunshine 
intensity. 


154 


129 


92 


77 


1 II 


,., 


1 l& 


73 


90 


9G 


43 




100 


Averaqe daiKi relat'wa increment in 
stem heiqhT. 


MO 


104 


146 


141 


228 


14G 


- 


107 


42. 


48- 


59 




n3 


Averaqe da.t^ relative mcrementlw 
leaf- product. 


135 


12<^ 


153 


115 


£93' 


I4B 




ioa 


29 


43 


" 




us 































364 



F. Mebeill Hildebeandt 



TABLE III 
Two-week data for exposed stations, Coleman, Easton and Princess Anne 



COLEMAN 

s-week periods. 

EX POSED 5TA TIOM. 


MAY 

MAi 
23 


MAY 
28 

JUNE 
1 1 


JUNE 

II 

JUKE 

24 


JUNE 

zi 

JULY 

a 


JUL1 

a 

JULY 
Z2 


JULY 

AUG 

5 


AUG; 
AUG. 

iq 


AUG 

19 
SEPT 

2 


SEPT SEPT 

5EPT3EPT 
Its SO 


3XFT OCT 
SO 13 

OC T. oc r 
13 26 


AV. 


Cu 1 ture n u m bar. 


1 


2. 


J> 


4 


5 


fa 


7 


a 


9 


IO 


1 1 


l£ 




Lenqth of qrowinq period, do^s. 


1*5 


19 


13 


14 


14 


11 


11 


1 4 


19 


14 


13 


1.5 




Numberr oj- plants. 


G 


<o • 


1 


4 


& 


<b 


3" 


fa 


4 


^ 


4 


<s 




Remainder bunitnut'ion index. 


412 


166 


133 


168 


S60 


^5>7 


S3S 


520 


3SO 


399 


- 


- 


na 


Averaqe doiKj relative ph\jsioloqieal 
temperature index. 


SI 


113 


IIS 


ISO 


170 


116 


158 


US 


S1 


BS 


- 


- 


123 


Averaqe daikj mean temperature, deq. rT 


Go 


72. 


67, 


79 


T9 


T6 


77 


76 


G7, 


G7 


~ 


- 


72. 


Average doilvj relative evaporation 
index. 


i39 


152 


137 


120 


143 


IJJ 


licj 


96 


J32 


I3S 


97 


93 


12.7 


Averaqe dailv rela+ive sunshine- 
in+ensirhf. 


Ml 


IfaO 


125 


Gl 


HI 


I £4 


MS 


&5 


- 


- 


- 


- 


120 


Averoqe cici K) relative increment in 
Stem heiqiit". 


19 


61 


107 


I3S 


wq 


I 57 


12.1 


107 


G2 


53 


G7 


2.S 


96 


Avcraqe dail^ relative increment in 
leaf- product. 


)OS 


III 


91 


172 


2.11 


19a 


itie 


t IO 


33 


" 


3a 


4 


IOT 



EA5TOM 

E- ujeek periods. 

EXPOSED STATiO/H 


MAY 
6 

MAY 

Z5 


MAY 
25 

JUNE 

e 


UUNE 

a 

jume 

Z2 


jurtE 
£2 

JULY 
fa 


JULY 
& 

JULY 

30 


JULY 
20 

AUG 

3 


AUG 

3 

AUG 

17 


17 
AUG 
31 


AUG 
31 

5EPT 
11 


SEPT 
"1 

SEPT 
zo 


SEPT OCT. 
2B II 

OCT OCT 
II Zfa 


AV. 


Cul+urc numboc 


' 


£ 


3- 


4 


-5 


e 


7 


e 


9 


IO 


It 


I£ 




Lenqth of qrowinq period, da^s. 


17 


11 


i-l 


I"* 


ii 


ii 


M 


11 


11 


H 


13 


15 




Number of plants. 


G 


4 


5 


G 


4 


s- 


S 


S" 


S 


<£> 


G 


3 




Remainder summation index. 


13o 


-1,53 


15f 


192 


53Z 


195 


Si 3 


JTI9 


392 


3BI 


32 to 


347 


MS 


Averacje dail\j relative physiological 
tern pe rature i ndex 


U7 


112 


MO 


i3l 


151 


1 33 


111 


116 


87 


76 


ei 


1S 


IO& 


Averaqe dail\j mean temperafure 5 deq.F 


65 


71 


7l 


71 


77 


71 


7G 


76 


G7 


GG 


61 


62. 


71 


Averaqc dailv, relative evaporation 
index. 


95 


130 


I53 


91 


Qfa 


I33 


i as 


II3 


1 1 I 


I04 


ee 


62. 


ios 


Averaqc Uail^j relative sunshine 
i n tensity. 


115 


172 


HI 


120 


12.1 


too 


H7 


99 


=37 


9J 


64 


* 


HI 


Averaqe dail^ relative increment in 
stem heiqht. 


15 


S7 


I IS 


132 


IIO 


116 


1 32 


US 


8> 


43 


65 


31 


9S 


Average dailv relative increment in 
leaf- produc+r 


71 


97 


LIZ. 


99 


ISZ 


ISO 


I63 


I29 


61 


35 


- 


. - 


IOS 



PRIMCE55 AHNE 

2- uj ee h pe rioclb 

Exposed static r-< 


may 

M 
MAY 
26 


MAY 

JUME 


B 

juriE 

23 


JUHE 
23 

JULI 

7 


JULY 
7 

JULY 

£l 


2I 

AUG. 
1 


1 

AUm 

IQ 


IS 
SEPT 


5eP7 
I 

SEPT 
15 


5EFT 
IS 

SEPT 
29 


SEPI 
29 

OCT 
12. 


OCT. 

12. 
OCT 

217 


AV. 


Culture number. 


1 


2 


3 


4 


s 


G 


7 


S 


9 


IO 


II 


12 




Lenqtb o[ qrowmq period, days 


15 


13 


IS 


11 


11 


11 


11 


It 


i1 


19 


13 


IS 




Number of plants. 


6 


5 


6> 


G 


G 


ta 


3 


fa 


fa 


S 


S 


■4 




Rcmaiuder Summation index. 


358 


-*o5 


501 


493 


51^ 


•J63 


5i3 


5ZO 


3B7 


3b3 


315 


32 S 


132 


Averoqe dailv relative pbvjsioloqical 
tern pb'rotu re i nde X. 


59 


105 


lOl 


1 32 


I44 


I2C3 


111 


1 46 


34 


71 


GO 


13 


IOI 


Avcraqe dai 1^ mea n temperaTure.deq.ff 


63 


70 


7S 


T\ 


7G 


71 


To 


'76 


67 


63" 


63 


Gl 


70 


Averaqe dail\j relative evaporation 

inde» 


117 


13-* 


- 


- 


t>3 


«3d 


lOl 


73 


73 


73 


70 


39 


Q4 


Averoqe daW^ relative, sunsnine 
i ntfen-sitAj. 


IOT 


It© 


103 


as 


as 


9& 


75" 


So 


79 


S2. 


&7 


^2 


86 


Averaqc dcuKj relative increment in 
stem beicjbT: 


62. 


no 


IOT 


12-1 


I«I7 


113 


ISS 


iat 


79 


*5" 


S3 


45 


I OS 


Averoqe dail^ relative increment" in 
leaf- product. 


96 


MS 


l 04 


MS 


IG3 


IOS 


I77 


I3S 


61 


31 


IS 


& 


9& 



Climatic Conditions of Maryland 



365 



TABLE IV 
Four-week data for exposed stations, Oakland, 



Chewsville and Monrovia 



OAKLAMD 

4--A>eck periods 
EXP05ED STATION 


MAY 

*» 

June 


June 
5 


rime 

ii 

JULY 
16 


JULT 

3 

JULY 
31 


JULY 

AUG. 
19 


JULY 

31 

*uq. 
Z7 


4UG 

M 
SEPI 

l£ 


■wc. 
2i 










AV^ 


Culture number, 


' 


z 


3 


9 


5 


<b 


7 


s 












Le-nqtb of qrowu7q period , dai^E>, 


27 


2S 


27 


za 


29 


27 


29 


29 












Number o r ' plants. 


4 


5 


JJ 


o 


-5" 


& 


& 


e 












Remainder summation index. 


663 


TSS 


765 


795 


BOO 


713 


719 


1=52 










73S 


AvGracjc daM^ relative phvjsioloqicol 
temperature index. 


66 


76 


es 


SJ 


77 


66 


65 


55 










7f 


Average dailu, mean temperature. deq.F" 


65 


OL-. 


« 


67 


67 


c5 


64 


62 










65 


Average duiKt relative cvaporatioo 
index. 


HG 


119 


69 


9a 


97 


tii 


64 


63 










94 


Averuqc da.l^ relative sunshine 
intensity. 


i2£ 


IIS 


lOfe 


MO 


I09 


06 


97 


82 










lO.j 


Average daiU relative increment in 1 
stem heiqnt. 


6b 


7a 


99 


69 


72 


ei 


63 


J7 










71 


Avcra^G da'rl^ i-eJative, increment m | 
leaf ar^a. 


4l 


<o7 


91 


75 


eo 


9o 


72 


32 










71 


A veracjc d ailv, relative incremcntin 1 
drv. wc'iqbt. 


o"l 


97 


io3 


as 


86 


S3 


T6 


46 










79 






























CMEW5VILLC 

1-week periods* 
EXPOSED STATION. 


riAv 

19 
JUME 

16 


jurtc 
jume 

JO 


JUME 

16 
JULY 


JUKE 

JULY 

26 


JULY 

19 

AUG 
1 1 


JULY 

26 

AUG. 
25 


Auq 
1 1 

StFT 

a 


AUG 

25 
5EP1 

22 


S7iPl 

8 

OCT. 

7 


SEPT 

22 

OCT 


OCX 

1 
NOV 

3 




AV. 


Cu l+ure nu mbcr 


1 


E. 


3 


9 


5 


6 


T 


e 


9 


to 


It 






Lenq+h of qrowinq period, da\is. 


26 


28 


ze 


£8 


28 


as 


2a 


ze 


29 


28 


27 






Number of plants. 


6 


fo 


4 


& 


s 


4 


3 


J 


6 


61 


6 






Remainder summation index 


as 1 ) 


SIS 


925 


976 


973 


96T 


953 


738 


650 


629 


4 90 




©23 


Averaqc dail^ relative pn^sioloqiaal 1 
Tern perature index. 


99 


113 


KG 


131 


131 


1Z-9 


129 


7& 


3"0 


52 


37 




c)G 


Averaqe daiKj meon "temperatLire,deq. ff 


TO 


It 


72 


tl 


79 


79 


73 


&e> 


61 


62 


SB 




69 


Averaqe. da d\j relative evaporation 
i iid^x. 


H3 


••'. 


91 


qz 


98 


96 


93 


so 


76 


69 


71 




90 


Averaqe da'il^ relative sunshine 


1 25 


12.3 


„. 


1 19 


ioa 


96 


B3 


86 


89 


59 


56 




9J 


Averaqe dculsi relative increment in 1 
stem heiqnt. 


1-S 


103 


122 


^1 


91 


64 


&4 


SO 


41 


4 1 


31 




IS 


Averaqe dailv. relative increment in 1 


73 


91 


132 


©3 


89 


94 


99 


37 


44 


45 


12 




79 


Averaqe daik re-latwe increment in 1 
O r ^ v»eiqlit 


IS 


IO£ 


142 


BT 


92 


S6 


&e> 


62 


51 


38 


24 




76 




























MONROVIA 

A «ceh periods, 
EXFOSED STATiOrt 


MAY 

18 

>uitE 

i5 


JUNE 
2S 


JUNE 
IS 

JULY 
13 


juris 

29 

2T 


JULY 
IS 

AU<J 
IO 


JULY 
■27 

AUG 
2*) 


AUG. 

to 

SEPT 

7 


AU<5 
24 

3EPT 
21 


3EFT 

7 
OCT. 

a 


3EPT 

21 
OCt 

19 


OCT 

a 
rxov 

2. 




AV. 


Culture number 


1 


e 


3 


9 


J 


S 


T 


e 


9 


IO 


" 






Lenqfb of qrowinq • period, daij-s 


ee 


ee 


ee 


ee 


2© 


He 


28 


£8 


31 


28 


25 






Mumbtir o^ plants. 


3 


3 


& 


9 


G 


3 


& 


3 


6 


6 


& 






Remainder summation index. 


i7& 


932 


932 


9SO 


999 


979 


9<H 


an 


770 


646 


963 




851 


Averaqe daily relative pbn&ioloqical 
temperature index 


IOS 


117 


"S 


134 


137 


131 


12. J 


82 


S£> 


56 


53 




IOZ 


Averaqe dailv^ rnGon temperature,deqF 


- 


72. 


72 


79 


IS 


14 


73 


66 


&Z 


©3 


SS 




6-^ 


Averaqe dail^ relative evaporation 

indc* 


- 


1-99 


12.3 


105 


..? 


1 IG 


90 


82 


89 


79 


81 




lOl 


Averaqe dail^ relative sunshine 
inte.nsit\j 


(37 


M3 


103 


106 


iob 


I02 


OO 


94 


do 


S9 


5fl 




96 


Averaqe daii^ reiaTive increment In 
stem heiqht 


63 


78 


8 4 


84 


7a 


91 


84 


Sb 


49 


41 


zd 




66 


Avcroqe dallu relative increment in 
leuf orea 


71 


iO0 


68 


99 


71 


jOfl 


7J 


TO 


39 


43 


12 




71 


Avcroqe da.lu relative increment in 
drq vvQiqnt, 


70 


i i*l 


lOfi 


99 


a9 


I&5" 


81 


87 


41 


48 


24 





79 



366 



F. Merrill Hildebeandt 



TABLE V 
Four-week data for exposed stations, College, Baltimore and Darlington 



COLLEGE 

4-weck ' periods. 

EXPOSBO STATlO^ 



Culture number. 



Length of qrowinq period,daxjs. 
dumber of- plants. 



Remainder SLimmaHon Indc^. 



tZ^r^Ut?^. p-s»-.oi«..-cui- 



Averaqc doiU) mcuu feinperohire, Jcq. K 



fl «™3 c d °' K > rclot.ve evaporation" 



sSS^ai'M ^'^'"c increment- i,, 



I^Jt- a S?ca dt "^ rolu "^ 'Mcrcniontio 



^Wa^,^ S re|uh<g , ncremaBt , ,„ 



BALTIMORE 



Culture lumber 



Lenqti? of qro^inq period, da s5 
Number of plants. 



Remainder summation indax. 



/Wage dall, meon-hamjaralura.deq F 



"mdeT da ''^ re "*>"»«porot,o»- 



aqe da.lx. relative 
^n3i7\j. 



"^^h?.?fi(. """*"* "—ept ,„" 



"^^q^TKj relaW incr5; ^ 5;n _. 



^Sr^S!^^ 



JULI 

17 
AUG 



AUG 
SEPT. 



AUG. 
£7 
SEPT. 




DARLIMGTOM 

1-ujeek periods 
Expq 5ep station. 



Culture number 



Lenq+h of qrow.nq period, dcn Js . 



Number of plants. 



Remainder summation index. 



_^raqad QI |. i mean temperature.^! 



*irtd£ e da, 't rslo1 "~ e ""PoraTioiT" 







Climatic Conditions of Maryland 



367 



TABLE VI 
Four-week data for exposed stations, Coleman, Easton and Princess Anne 



COLEMAN 

4- week periods. 
EXPOSED STATION. 


13 

ii 


1AH 

JUNE 

-■I 


IUL.V . 

<> 


UMI 

24 
ULY. 


JUkf JUUVAUQ AUG 3EFr. 

8 Z2 S 19 £ 
AUG AUG, SEPT SEFT 3EPT 
S 19 z. IG 30 


DEPT 

IG 
OCT. 


.111 

ZO 
OCT. 
2CJ 


AV. 


Culture number 


i 


C 


3 


1 


5 


S 


7 


e 


9 


IO 


II 






Lenqt" °{ qrowinq period, cla\js. 


=9 


zi 


.-' ( 


£6 


ze 


2S 


ee 


28 


ze 


27 


26 






lumber of plants- 


G 


& 


1 


1 


G 


G 


^ 


G 


s 


4 


37 






Remainder sutnmah'on index. 


OIO 


B99 


■>2l 


04s 


OT7 


OSS 


050 


qio 


709 


' 


" 




939 


Average daily relative. piT\|sIo]oqical 
temperature index. 


9T 


II& 


121 


ISO 


isa 


ISE 


133 


to 


85 


- 


" 




IZS 

73 


Averaqe daily mean tcmperciture.,dcq. f: 


G9 


io 


II 


17 


Ttl 


77 


77 


72. 


67 


- 


- 




Averaqe dai Im relative eva poraticm 
index. 


i-io 


145 


iaq 


132. 


119 


i a r 


ioe 


I 14 


134 


116 


I S 




I2B 


Averaqe daily relative sunshine, 

in+cHSit-j. 


iSi 


113 


93 


IOI 


iiJ 


I2I 


102 


- 


- 


" 


- 




I2.I 


Averaqe dail-y relative increment in 
stem lieiqiit; 


6') 


Bl 


94 


lOG 


IOQ 


1 09 


B4 


72 


S3 


SO 


53 




ao 


MveAjc\c daiVy relative increment in 
leaf area. 


116 


ISO 


1ST 


111 


I3S 


1-4 5 


ioe 


89 


71 


- 


MT 




I IT 


Averaqe daily relative increment in 
dry, w » qiit 


IZG 


1 53 


159 


143 


I34 


H6 


83 


113 


oT 


75 13 




(13 






















£A5TOrS 

1-u;eeV\ periods. 

EiPObCU JTATlOM. 


S 

a 


JUNE 
22 


jura 
a 


juru 

JULY 
ZO 


JUL.Y 
to 

AUG, 

3 


ZO 

AUG. 
17 


AUS. 

AUG. 

31 


AUG 
17 

AUG 
11 


AUG. SCF7 
31 11 

sEPt ocr. 
26 II 


5EP1 

^a 

OCT. 
ZG 


OCT. 

nov. 


AV. 


Culture number 


i 


2 


3 


4 


3 


£> 


7 


8 


9 


IO 


II 


1 2. 




Lenqth oj-qrowioq period, daqs 


31 


28 


ze 


26 


ze 


36 


28 


ze 


SO 


Z7 


2e 


26 




Mumber- of plante- 


& 


4 


^ 


G 


1 


3" 


37 


5 


s 


G 


& 


■3 




Remainder summation, index. 


e»9 


901 


941 


|OZ1 


lOi7 


100a 


1032 


9H 


T75 


707 


G73 


■499 


SM 


Averaqe dailvj relative pbijaio/oqical 
"temperature, index.. 


go 


„, 


12.1 


113 


111 


139 


1137 


II7 


82 


70 


3"G 


39 


i03 


Averaqe daiN mean "te.mpcrature,dcq. r~ 


tfe 


Tl 


73 


7G 


76 


73" 


To 


72 


G7 


G3- 


G3 


3"G 


70 


Averaqe doilv relative evaporation 
index. 


115 


132 


ME 


89 


no 


131 


I2I 


I 12 


IOS 


96 


S3", 


93 


iOR 


A vc r-a qc ciu'i l ^ rel at i\e su nshi ne 
in+ensit^. 


isq 


IS7 


131 


\Z2. 


I IG 


113 


IOS 


ge 


■9G 


eo 


sa 


GO 


I03 


Averaqe daily relative increment in 
s,tcm heiqlir. 


it 


72 


91 


I03 


103 


1 13 


I OJ 


ei 


J6 


JO 


3"G 


30 


75 


Averaqe daily relative increment" in 
leaf area. 


T4 


T^, 


,1, 


qo 


IOI 


121 


I2I 


93" 


GG 


U.-1 


3d 


8 


82 


Average daily relative increment in 
drq weiqht. 


SO 


SI 


119 


9i 


IIS 


■ 2.1 


i 21 


ei 


72 


1a 


32 


22 


83 








PRIMCE55 /\nriE 

1-ujeek periods. 

E.X POSSD 5r^TIO/S 


i-iav 

juru 
o 


MAY 
Z3 


JUNE 

JUU 
7 


dimf 
23 


7 


AUG 

ia 


sei'r 


ia 

5EP 

IS 


SET 

SEP 
29 


S£ P 
IS" 

ocr 
ia 


5Ef 
29 

ocr 
27 




AV. 


Culture numben 


1 


z 


3 


*l 


5 


t? 


7 


e 


9 


IO 


II 






Lenqth of qrow w?q period, days. 


zs 


ze 


£9 


ze 


za 


aa 


za 


£S 


aa 


27 


ze. 






Number of plants. 


1 g 


5 


.5 


» 


G 


fa 


3 


G 


& 


S 


& 






FlCiiiainclcr summation index.. |7&3 


909 


997 


IOO" 


997 


99fa 


1 033 


=?OT 


730 


678 


G43 




sao 


Averaqe da\Kj relative phijsioloqicul 
temperature "index. 


£< 


I02 


117 


is a 


I3C 


136 


113 


I IO 


78 


GG 


52 




106 


Averaqe daily mean temperature, deq.F 


61 


73 


IS 


7J 


73 


"5 


7G 


72 


fc>6 


G1 


G2 




7 i 


Averaqe daily relative evaporation 1 

indev 


126 


- 


- 


- 


73 


92 


90 


7S 


7e 


74 


55 




83 


Averaqe daily relative sunshine 
intensity 


U.7. 


,„ 


94 


07 


9Z- 


BO 


ei 


S3 


ei 


73- 


S3~ 




67 


Averaqe daily relative increment in 
stern helq lit. 


o 


ee» 


103 


IO". 


i3a 


iaj 


1 36 


ri4 


&3 


3b 


1^ 




92 


Averaqe daiJtj relative increment in 
leaf **r&a. 


tz.3 


\S5 


no 


- 


1 27 


™ 


ZC*. 


IOE 


->■■• 


72 


36 




II7 


Averaqe daily relative increment "m 
dry weiqljt: 


!._'■._ 


JOC 


137 


i If 


I37 


133 


I7I 


1 ? 


a > 




e.7 


38 




HG 



368 



F. Merrill Hildebrandt 



TABLE VII 

Data for covered stations, Oakland and Baltimore 



OAKLAMD 

5-week periods. 

COveRED ^>T^-riQr*4. 


MAY 
22 

JUNE 

4 


JUNE 

4 

JUNE 

ia 


JUflE 
16 

JULY 

2 


july 

z 

JULY 

15 


IS 

JULY 
30 


JULY 
30 

AUG. 
13 


AUQ. 
1.5 

AUG- 
26 


AUQ. 

SO 

sept 


5EPT 

3EPT 
24 








AV 


Culture number 


1 


z. 


Z> 


•q 


3 


£> 


7 


& 


9 










Lenqth of qrowinq period, daus. 


13 


ia 


14 


\2> 


|3 


1-4 


13 


iG 


i3 










dumber of plants. 


s 


<3 


fc 


3 


e? 


- 


3 


3 


3 










Ayeraqe daiKj relative evaporation 
index. 


nz 


147 


133 


log 


Hi 


112, 


i ia 


85 


93 








120 


Averaqe daiKj relative increment 
in stem heicjlffc 


so 


112 


DO 


12.1 


ICO 


- 


S4 


TO 


5& 








92 


Averaqc daiKj relative irrcrement 
iii leaf -product: 


IS 


85 


12.7 


96 


134 


- 


B9 


<34 


9 








T8 



OAKLAND 

4-week periods. 

COVeR&D STATION. 


MAY 

JUNE 
IS 


JUNE 
4 

JULY 
2 


JUNE 
10 

JULY 
15 


JULY 

2 

JULY 
30 


JULY 

t5 
AUG. 

13 


JULY 
30 

AUG.. 
2fo 


AUG 
13 

5EPr 

n 


AUG, 
2fi 

SEPT. 
Zf\ 










AV. 


Culture number 


1 


2 


3 


4 


5 


6 


7 


8 












Lenqth of qrowincj period, da^s. 


27 


28 


Z.T 


ZG 


29 


27 


sq 


2.9 












dumber of plants. 


S 


6 


e 


5 


6 


- 


3 


-5 












Ayeraqe daiKi relative evaporation 
i a d ex . 


l&O 


190 


izj 


1 13 


\ 15 


* 
HZ 


99 


79 










in 


Averaqe daily relative increment in 
stent taeiqhlr. 


56 


Si 


ee 


ioe> 


°n 


- 


03 


so' 










61 


Averaqe daiKj relative increment \n 
leaf area. 


- 


107 


145 


155 


1 IS 


- 


- 


66. 










l 16 


Ayeraqe dail^j relative increment 
in drq weiqhh 


57 


9T 


12.1 


123 


... 


- 


80 


59 










93 



BALTIMORE. 

j-wecK penoas. 

CQVERED STATION. 






JUNE 
IO 

JUNE 
£5 


JUNE 
ZS 

JULY 

. 9 


JULY 
9 

JULY 
23 


JULY 
23 

AUQ. 
& 


AUG 

6 

Aua 
20 


AUG- 
20 

SEPT 

3 


SEPT 

3 
SEPT 

19 


SEPT 
19 

OCT. 


OCT. 

OCT. 

14 




AV. 


C u 1 1 u i^e number: 






3 


4. 


3 


& 


7 


e 


9 


\o 


1 1 






Lenqrh of qrowinq period, days. 






15 


H 


11 


14 


14 


14 


16 


12. 


13 






Number of plants. 






& 


6 


4 


6 


J 


J3 


3 


.5 


•A 






Averaqe clailvj relative evaporation 
i ndex . 






U9 


9-5 


112. 


log 


I l 7 


ST 


SI 


TT 


73 




97 


Averaqe dail^ relative incremenif 
In stem he'iq n/n 






157 


i&o 


326 


233 


197 


191 


36> 


70 


155 




172 


Averaqe daiKj relative increment" in 
leaf - product. 






1 1 1 


150 


eoz 


W7 


ieo 


IS3 


51 


21 


■43 




125 



BALTIMORE 

4-weck periods. 
COVERED 5TATIOJ-H. 




MAY 
29 

JUNE 
25 


JUNE 
IO 

JULY 
9 


JUNE 
25 

JULY 
23 


JULY 

9 

AUG. 

<3 


JULY 
23 

AUG". 
20 


AUG 

3EPT 

3 


AUG; 

zo 

5Epr 
19 


SEPT 

3 
OCT, 

1 


SEPT 

19 
OCT 

14 


OCT. 

OCT 
3» 




A.V. 


Culture number 




z 


3 


4 


3 


6 


7 


8 


9 


IO 


H 






Lenqth of qrowinq period, day3. 




2.7 


29 


Z6 


2S 


2S 


2© 


30 


z& 


25 


30 






Number of plants. 




S 


b 


& 


A 


& 


3 


3 


3 


3 


■4. 






Averaqe dai Ki relative evaporation 
index. 




119 


IOT. 


107 


11) 


t 13 


102 


84 


79 


73 


70 




97 


Ayeroqe daiKj relative increment 
in s re m hefq ht- 




156 


I2S 


134 


231 


IG9 


153 


131 


72 


I03 


78 




I3& 


A ve raqe d a 1 Kj re 1 a ti ve inc re m <s.\-& 
in leaf area. 




" 


-- 


>&6 


23Q 


137 


I'M 


l-S"i 


ri 


IOI 


- 




154 


Averaqe daily relative increment 
m dry weiqrir. 




(34 


5*1 


1 IO 


163 


103 


135; 


>14 


6^( 


SO 


3i 




9S 



Climatic .Conditions of Maryland 



369 



TABLE VIII 

Data for covered station at Easton and for forest station at Baltimore 



EASTorn 

2-weeK periods. 
COVERED 3TATIOM, 




MAY 
JUNE 

a 


JUNE 

a 

JUNE 
22 


JUNE 
22 

JULY 
6 


JULY 

JULY 
20 


JULY 
ZO 

AUG 
3 


AUG 
3 

AUG 

n 


AUG 

17 

AUG 

31 


AUG 

31 
5EPT 

11 


SEPI 

11 

SEPT 

es 


SLIT 

za 

OCT 
1 I 




AV. 


Culture number 




z 


3 


4 


J> 


& 


7 


& 


9 


to 


1 1 






Lenqtlz op qrovv'mq period, da^s. 




11 


14 


11 


H 


14 


11 


14 


14 


14 


14 






Number of plants. 




A 


e 


■1 


3 


(3 


& 


J 


6 


fa 


e> 






Ayeraqe dailvj relative evaporation 
index. 




1 66 


135 


SO 


I I2 


i-m 


is9 


I3i 


131 


104 


IO0 




12.4 


Averaqe daily relative iiicrenicnhn 
stern heiqkr. 




i 29 


i ie 


1" 


n t 


|77 


177 


\&S 


<?o 


9C> 


" 




145 


Averaqe daiKi relative increment 
ia leaf- product 




IZ.Z 


.13 


eq 


ISO 


ies 


ZIO 


ZZ5 


13.5 


55 


- 




(4Z. 



LtASTOfH 

4- week periods. 

COVERED 5TAriON, 




MAY 

JUNE 
Z-Z 


JUHE 

a 

JULY 


JUNE 
22 

JULY 
SO 


JULY 

£. 

AUG- 

3 


JULY 

ZO 

AUG. 
17 


AUG. 

3 

AUG 
31 


AUG 
17 

5EPT 
14 


AUG 
51 

SEPT 
2S 


SEFT 

14 
OCT. 

1 1 


SEPT 

ze 

OCT 
£6 


OCT 

MOV. 
fa 


AV. 


Cu 1 tui'C number 




Z 


3 


4 


5 


6 


T 


e 


9 


IO 


U 


12. 




Lenqth o[ qrowi'nq pen ocl* daws. 




z.e 


ze 


as 


ze 


ZS 


28 


28 


2& 


£7 


ZS 


26 




Number of plants 




4 


6 


& 


- 


6 


° 


•S - 


6 


fa 


~ 


- 




Averaqe daiivj relative evaporation 
index. 




iSl 


loe 


qG 


• 


137 


130 


I3t 


US 


«OG 


icjA 


n& 


I2.0 


Averaqe daiivj relative increment 
in stem he'iq^t. 




103 


12a 


12-5 


- 


134 


IS& 


125 


75 


ei 


- 


- 


H6 


Averaqe daiKj relative, increment 
in lea f ar~G.a. 




1 17 


i9' 


90 


- 


138 


202 


1<S9 


104 


es 


- 


- 


137 


Averaqe ctaiKj relative increment 
"hi dv~y weight". 




108 


144 


ee 


- 


107 


HO 


1^1 


tos 


5(5 


- 


— 


109 



BALTIMORE. 

2-week periods. 
FOREST 5TAT fOM. 








JUhE 

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370 F. Merrill Hildebrandt 

The 2-week tables for the exposed stations (tables I— III) show in line 5 
the remainder-summation temperature index for each culture period, this 
being obtained by subtracting 39° from each daily mean and then summing 
the remainders for the period. Line 6 gives the average daily relative 
physiological index for each period. Line 7 gives the average daily mean 
temperatures for each period, line 8 the average daily relative evaporation 
index, and line 9, the average daily relative sunshine-intensity value Line 
10 shows the values of the average daily relative increment of stem height, 
and line 11 the values of the average daily relative increment of leaf-product. 
The two-week tables for the covered stations correspond to the two-week 
tables for the exposed stations, except that no temperature or sunshine data 
are here available and the tables thus contain only the relative evaporation 
indices and the two plant values. This is also true for the Baltimore forest 
station. 

The 4-week tables correspond, line for line, with the 2-week ones, except 
that the 4-week tables show the average daily relative increment of leaf 
area (instead of the average daily relative increment of leaf-product) and a 
line is added to the 4-week tables giving the average daily relative increment 
of dry weight. Each 4-week value of the relative daily physiological temper- 
ature index, the relative daily evaporation index, and the relative daily 
sunshine intensity, was obtained by averaging the relative values of these 
climatic factors for the 2-week periods in question. The 4-week value 
of the remainder-summation index for each period was obtained by adding 
the values of this index for the two 2-week periods that make up the 4-week 
period under consideration. The average daily mean temperature for the 
longer periods was obtained by taking the mean of the two average daily 
means for the two 2-week periods involved. 

It will be noted that the plant values are uniformly given at the bottom 
of the table, with a double rule separating them from what precedes. 

Figures 2-6 present graphically certain of the data given in tables I-VIII. 
Graphs for plant values are denoted by black lines and those for climatic 
values are shown in red. In all of these graphs the ordinates represent 
magnitudes of the plant and climatic relative values and the abscissas repre- 
sent the time of the year. The ordinate scale is given at the left of each 
set of graphs, for convenience of reference, and the dates of the beginnings 
of successive culture periods are shown on the base line. Thus, for the first 
2-week period at Oakland, the ordinates show the average daily relative 
values of the plant and climatic measurements for the 2-week period begin- 
ning May 23. The 100-line of the ordinate scale represents the average 
seasonal value for all stations (as previously noted) , this being the unit used 
in expressing the corresponding relative values. Full black lines (appearing 
only on 4-week graphs) represent dry weight. Dash black lines represent 
height. Dotted black lines (only on 4-week graphs) represent leaf area. 



Climatic Conditions of Maryland 371 

Dash-and-dot lines (only on 2-week graphs) represent leaf-product. Full 
red lines represent temperature. Dash red lines represent evaporation. 
Dotted red lines represent light. 

The results obtained will now be brought forward, with some discussion, 
which is to be read with reference to tables I-VIII and figures 2-6. 

Results for Stations in the Open 

The data for the stations in the open will be considered as of two main 
groups, the 2-iveek data and the 4-week data. 

THE 2-WEEK VALUES 

The 2-week -plant data for stations in the open (see figs. 2 and 3, black lines) 

As has been stated, the plant measurements here in question were taken 
about two weeks after planting and included stem height and leaflet dimen- 
sions. From these have been derived (1) the relative mean daily rate of 
increase in stem height per plant and (2) the relative mean daily rate of 
increase in total leaf-product per plant, both for each 2-week period. 

Therefore, one of these 2-week plant values represents the stem-producing 
power of the plant and the other stands for its leaf-producing power, under 
the given set of external conditions acting during that period. Since the 
plants are taken to be alike at the start, (seeds) these two derived plant values 
should be the same for all individuals if all were subjected to the same effect- 
ive environmental conditions throughout the period, and when the various 
plants are exposed to different environments the values just mentioned 
become criteria by which the effectiveness of one environment may be com- 
pared with that of another, with reference, of course, to the particular set of 
internal conditions represented by the plants at the beginning of the tests. 
The two plant values just mentioned may thus be regarded as relative meas- 
ures of the effectiveness or efficiency of the environmental complex for the 
2-week period considered, as it acted to produce stem elongation and leaf- 
product increase, upon the soy-bean plants employed in this investigation. 
For convenience, the following discussion will refer to the graphs (figs. 2 and 
3) rather than to the tables, but tables and graphs both present the same 
data in every case. This discussion will be given under two headings: (1) 
Correlations between the two plant graphs and (2) Trend of the plant 
values and their seasonal averages for the various stations. 

Correlations between the two 2-week plant graphs. — It is readily seen that 
the two graphs showing relative rates of increase in stem height and in leaf- 
product agree in their general direction of slope from period to period, through- 
out the season and for all stations. In many cases the two plant graphs not 
only slope in the same general direction (upward or downward) but their 






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Fig. 2. Graphs of 2-week data for exposed stations, as named. 

Black: Height, ; Leaf product, — ■ ■ • — 

Red: Temperature index, ; Evaporation index, ; Sunlight 

index, 

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Fig. 3. Graphs of 2-week data for exposed stations, as named (continued). 
(Lines as in fig. 2.) — Graphs of 4-week data for exposed stations, as named. 

Black: Dry weight, ; Height, ; Leaf area; 

Red, as in fig. 2. 

373 



374 F. Merrill Hildebrandt 

corresponding angles of slope are nearly the same and their corresponding 
ordinates are about equal, so that they nearly coincide for considerable por- 
tions of their length. In other words, there appearsto have been a pronounced 
general agrement between the effectiveness of the environment to produce 
stem elongation and its effectiveness to increase the magnitude of the leaf- 
product, as shown by these cultures. If this agreement were perfect it 
would mean, of course, that the environment exerted the same influence 
upon the process of leaf-surface increase (as measured by leaf-product) and 
upon the process of stem elongation, and either of these two criteria would 
be a measure of the other. But the coincidence of the two graphs is not by 
any means perfect and it becomes a matter of interest to study their differ- 
ences, as shown by the corresponding relative values of their ordinates. 

Inspection of the graphs shows that, leaving those for Oakland out of 
account, the index for stem height increase is frequently greater than the 
other plant index for the early and late portions of the frostless season, and 
that this relation generally is reversed for the middle portion. In other 
terms, the graph for stem elongation generally lies below the other graph 
for the middle of the season and above it for the beginning and end of the 
season. In still other words, the seasonal maxima of leaf-product values are 
generally relatively higher than those of stem elongation, while the seasonal 
minima of the former are lower than those of the latter. It may be stated, 
as an approximation, that when these two plant values are both about 100 
(as the data are presented in this paper) the leaf-product value is generally 
the higher of the two, while when both are below 100 the elongation value is 
usually the higher. In the case of Oakland, both values are comparatively 
very low throughout the season and, while the index of stem elongation reaches 
somewhat above- 100 for two periods, this index is never surpassed in magni- 
tude by the index of leaf-product increase. 

The generalization just stated indicating a relation between the rates of 
two plant processes, seems to be a physiological one, dependent upon the 
nature of the soy-bean plant and hence largely predetermined by the internal 
conditions of the seed. Within the range of environmental conditions en- 
countered in this study it appears that the taller and more leafy the plant 
becomes in the first two weeks of growth, the lower is the value of the ratio 
of final height to final foliar expanse. The two growth processes here consid- 
ered are, therefore, clearly interrelated and neither one alone is to be regarded 
as a criterion of plant growth in general. The average of these two indices 
may be considered as a tentative index of the general growth of the plants 
during the first two weeks from the seed. Inspection of the 2-week graphs 
leads to the impression that this mean of the two values offers perhaps the 
most promising way to obtain from them a single index of plant growth. The 
two are always so nearly parallel throughout the season (nearly coinciding 
for many periods, as has been stated) that the charts have not been further 



Climatic Conditions of Maryland 375 

complicated by introducing the graph for the average, but the form of this 
graph is readily appreciated from the two graphs that are given. 

The general relation between the two plant values that has just been 
emphasized does not always hold, and the more detailed discussion of the 
plant graphs for individual stations, given in the next following paragraph, 
is of value in showing the main exceptions. 

For Oakland the height value lies above that for leaf-product throughout 
the season. The two graphs have the same general direction of slope except 
for the period beginning July 16. For Chewsville the height graph exhibits 
the same general direction of slope as does the leaf-product graph, from 
period to period, throughout the season, with the former well above the latter 
for the last four periods (beginning August 25, September 8, September 22 
and October 27). For the periods beginning June 16 and June 30 the latter 
relation also holds, although the index values are large, especially in the case 
of the first of these two periods. For Monrovia the two plant graphs follow 
each other very closely throughout the entire season. The graphs for Col- 
lege for the periods beginning July 3, July 17 and July 31, illustrated the tend- 
ency of the height values to decrease relatively to those of leaf-product when 
both values are large. For the periods beginning September 10, September 
25 and October 10 for this station, both values are small and, as would be 
expected, stem height is relatively greater than leaf product. For the period 
beginning June 19 the expected relation between the two graphs does not ob- 
tain. For Baltimore the periods beginning June 10, July 23 and August 20 
are exceptions to the proposition that the height graph should he below the 
graph of leaf -product when both plant values are large. The generalization 
is true, however, for the remaining periods of the Baltimore season. For 
Darlington the two plant graphs agree closely in value throughout the season. 
For Coleman stem height and leaf-product show the expected relation. For 
Easton the generalization holds, with three exceptions: the height index is 
lower than the other for the period beginning May 8, although both indices 
have low values, and this relation is reversed for the periods beginning June 
22 and July 20, in spite of the fact that both values are large in these cases. 
For Princess Anne the graphs show values of the height index higher than 
those of the other index for the periods beginning June 23, July 7 and July 21, 
although both indices are large for all three periods. Otherwise these graphs 
agree with the generalization. 

The fact that the generalization given above" holds in the great majority 
of the cases here studied renders the exceptions of special interest. Assum- 
ing that the seeds were all alike at the beginning of all cultures and that no 
disturbing influence was introduced by soil conditions, it may be supposed 
that the periods characterized by exceptions to this generalization should also 
be characterized by some sort of corresponding peculiarities in the aerial 
environmental complexes. Now, a study of the charts for the exposed sta- 



376 ■ F. Merrill Hildebrandt 

tions brings out the following fact: most of the 2-week periods for which 
both plant values are large and yet the index of stem elongation is greater 
than that of leaf-product increase, are characterized by low indices of sun- 
shine intensity. This suggests that the plants of these cultures experienced 
an acceleration in their rates of stem elongation due to low light intensity, in 
short that they exhibited some of the effects of incipient etiolation. They 
seem to show a somewhat increased rate of stem elongation and a somewhat 
decreased rate of leaf expansion, as compared with plants receiving more 
radiation. This interpretation is not to be regarded as at all well established, 
but it is at least a suggestion of one way in which the external condition of 
light intensity and duration may be registered in such plants as were here 
employed. 

Trends of the 2-week plant values and their seasonal ranges for the several 
stations. The following consideration of the seasonal marches of the 2-week 
plant values for the various stations will be limited in extent, since most of 
the facts and deductions that seem to be of importance in this connection 
can be better brought out later. Attention will here be called only to two 
characteristics of the plant. graphs: (1) They begin with values of about 100 
rise to high midsummer values and then fall to low values at the end of the 
season. (2) Differences in the magnitude of the midsummer maxima con- 
stitute the chief differences between the graphs for the various stations. 

Oakland shows lower values for stem height and leaf-product than does 
any other station, due largely to the low temperatures prevailing at this 
station throughout the season. The data of this study indicate that the 
climate of Oakland, so far as it affects the plants, is very unlike that of any of 
the other stations employed. Both plant graphs for this station show the 
typical low values at the beginning and end of the season, however, with 
midsummer maxima of 124 for stem height and 89 for leaf-product. Chews- 
ville shows typical graphs, the highest value reached by stem height being 
143 while the leaf-product maximum is 139. The end of the season at this 
station is characterized by very low leaf-product values. The graphs for 
Monrovia are also typical, with low values of both indices for the period begin- 
ning May 18 and low values at the end of the season, after midsummer 
maxima of 121 (stem height) and 138 (leaf-product). An explanation of the 
low values shown for the Monrovia periods beginning September 7 and 
October 8 may he in the fact that a minimum temperature only several degrees 
above freezing was reached during each of these periods. For both Chews- 
ville and Monrovia the plant values are, for the most part, lower than 100, 
with relatively low midsummer maxima of 132 and 203, for stem height and 
leaf-product, respectively. The Baltimore plant graphs begin with high 
values and reach maxima of 183 and 233, for stem height and leaf-product 
respectively. For Darlington the main distinguishing features are the very 
high maxima of 228 for stem height and 295 for leaf-product, for the period 



Climatic Conditions of Maryland 377 

beginning July 10, and the relatively high values shown by the graphs for the 
beginning of the season. The midsummer maxima for Coleman are 157 
(stem height) and 211 (leaf product). The plant graphs for Easton show 
relatively low values of the midsummer maxima, 146 being the highest value 
reached for stem height and 163 for leaf-product. Also, the plant values 
are low for the beginning of the season for this station. The midsummer 
maximum for stem height for Princess Anne is 197 and the corresponding 
maximum for leaf-product is 163. 

The plant graphs, as may be seen from the above outline of their main 
features, fall into three groups: (1) The Oakland graphs, which show values 
of the leaf-product index below 100 for all periods and similar low values of 
the stem-height index for all periods except those beginning June 14 and 
August 14, while the maxima of these graphs are relatively low. (2) The 
graphs for Chewsville, Monrovia, Princess Anne and Easton, showing higher 
midsummer values of the plant growth-rates than do the Oakland graphs, 
the maxima being about one and one-half times the seasonal average. (3) 
The graphs for College, Baltimore, Darlington and Coleman which are distin- 
guished by high relative values of their maxima. This classification serves to 
summarize such characteristics of the graphs as are of present interest. 

The 2-week climatic data for stations in the open (see figs. 2 and 3, red lines) 

The 2-week climatic data consist of the average daily relative values of 
the indices for temperature, evaporation, and light for each of a series of con- 
secutive periods extending through practically the entire growing season, 
each period being about 14 days long. These values therefore furnish a con- 
tinuous record of the growing season at each station. The 4-week periods, 
however, overlap, each one including the last two weeks of the preceding and 
the first two weeks of the following 4-week period, so that the cUmatic aver- 
ages based on the 4-week data form a smoother curve than do the 2-week 
values in every case, small variations in the conditions being to a great extent 
obscured by, averaging the overlapping periods. This series of 2-week values 
therefore exhibits the march of the climatic conditions at each of the vari- 
ous stations in somewhat greater detail than do the corresponding series of 
4-week values. The former will therefore be made the basis for a somewhat 
detailed and comparative discussion of the climatic conditions at the various 
stations, temperature receiving attention first and light and evaporation being 
afterwards considered together. In each case, the general characteristics 
(common to most or all of the stations) of the seasonal march of the condition 
considered will be brought out, after which attention will be given to peculi- 
arities of the values for individual stations. 



378 F. Merrill Hildebrandt 

The 2-week temperature data 

The most obvious general characteristic of the physiological temperature 
index is that its value is high for mid-summer and low for the beginning and 
end of the season, for all stations. Graphs of similar form are obtained when 
daily means and remainder summations are correspondingly plotted, but the 
midsummer rise is much more pronounced in the graph of physiological index 
values (here employed) than in either of the others. The second general 
characteristic of all the graphs of the physiological index of temperature is 
that each graph possesses two maxima, both of which have about the same 
magnitude. The first occurs for the last two weeks of July and the second 
for the last two weeks of August, this statement being true for all the stations 
considered except Oakland, for which station they both occur relatively 
early in the season, in the last two weeks in June and July respectively. A 
third feature which is common to most (though not all) of these graphs is that 
the upward slope is more gradual before the occurrence of the high midsummer 
maxima than is the downward slope after their occurrence. A generali?ed 
temperature-efficiency graph, representing averages of the corresponding 
values for all of the stations, is not symmetrical about the ordinate for its 
highest midsummer value; it slopes upward less rapidly than downward. 
A fourth general characteristic of these graphs lies in the fact that the final 
low index values of the frostless season are not very different for the various 
stations. The following consideration of the graphs for some of the individ- 
ual stations will serve to bring out the points mentioned above and will give 
opportunity to note exceptions to the general statements just made. 

With regard to the forms and other characteristics of the 2-week tempera- 
ture-efficiency graphs, the nine stations studied may be placed in five groups: 
(1) Chewsville and Monrovia, (2) Baltimore, Darlington and Coleman, (3) 
Easton and Princess Anne, (4) College, and (5) Oakland. These five groups 
are discussed in order below. It will be noted that groups 1, 2 and 3 are 
composed of stations that are located near each other, and this probably 
accounts for the grouping. 

. Chewsville and Monrovia. The graph of physiological temperature indices 
for Chewsville shows all the characteristics mentioned as general throughout 
the series of stations. It rises gradually during the first three periods, (per- 
iod beginning May 19 to period beginning June 16), then drops slightly during 
the fourth period (beginning June 30) after which it rises for the period begin- 
ning. July 14 to a primary maximum of 149. The value for the 6th period 
(beginning July 28) is relatively low (112), after which a secondary maximum 
(145) occurs for the period beginning August 11. The index value in question 
then decreases rapidly during the next two periods attaining a magnitude of 
48 for the 9th period (beginning September 8) and remaining low until the 
end of the frostless season. Monrovia has the same sort of graph as Chews- 



Climatic Conditions of Maryland 379 

eille, the maxima coming in the periods beginning July 13 and August 10. 
The minimum relative value of the temperature index is 53 for the period 
beginning September 21. 

Baltimore, Darlington and Coleman. For Baltimore, the physiological tem- 
perature values increase gradually to a primary maximum of 162 for the 
period beginning July 29. The secondary maximum occurs in the period 
beginning August 6, after which there is a relatively rapid decline of the 
index values, to 62 for the period beginning September 3. The Darling- 
ton graph has its first maximum in the first two weeks of July and its second 
in the 2-week period beginning August 7, and then falls off rapidly to a mini- 
mum of 46 for the first period in September. The graph for Coleman shows 
a gradual rise, two maxima for the periods beginning July 8 and August 5, 
and a rapid fall. The temperature record is incomplete at this station and 
the low values for the end of the season are not available. 

Easton and Princess Anne. For Easton there is a gradual rise to a maxi- 
mum of 154, for the first period in July, the second maximum coming in the 
period beginning August 17. The curve then falls to a minimum of 48, for 
the last period of the season. The Princess Anne curve shows the two typi- 
cal maxima in the periods beginning July 9 and August 18, with a minimum 
of 43 for the last period of the season. 

College. The College graph of physiological indices is unusual in showing 
a marked rise for the period beginning June 19, thus giving the graph three 
maxima (129, 148 and 143, for the periods beginning June 19, July 17 and 
August .14, respectively). The graph descends rapidly to a value of 50, for 
the period beginning September 25. 

Oakland. The temperature-index values for Oakland are all relatively 
low, being always considerably less than the seasonal average for all periods 
and stations. This graph shows two maxima, one for the latter half of June 
and the other for the latter half of July. Each of these maxima occurs about 
a month earlier than do the corresponding ones for the other stations here 
studied. The Oakland graph is also unlike those for the other stations in 
that its downward slope is more gradual. Its final relative value is 43, for 
the 2-week period beginning September 12, which was the last full period for 
this station before the occurrence of a killing frost. The most outstanding 
characteristics of the Oakland season, in respect to this temperature-effi- 
ciency graph, as compared with the seasons at the other stations, are: (1) 
general low values of the physiological temperature-index. (2) short duration, 
owing to the occurrence of late spring and early fall frosts, and (3) early 
occurrence of the maxima. These marked differences between the Oakland 
graph and those for the other stations here dealt with are no doubt largety 
due to the relatively high altitude of Oakland as compared with the others 
as has been mentioned by McLean in his comparative study of the Easton 
and Oakland seasons based on these same data. 



380 F. Merrill Hildebrandt 

The generalized graph. Leaving the graph for Oakland out of account, 
those for the other stations may be described as a single generalized graph, 
in the following general terms. Beginning with a relative index-value of 
about 80 (for the first part of May) the graph rises to a maximum (about 
150) for the first part of July, falls slightly and rises again to a second maxi- 
mum of about the same value as the first, for the first part of August, and 
finally falls to a minimum value of about 50 for the last period of the frost- 
less season. That the initial values are not lower is no doubt due to the 
fact that the cultures were not started until somewhat after the beginning 
of the frostless season. (See McLean's paper, already cited.) This gener- 
alization of the temperature values for the various stations is not, of course, 
to be considered otherwise than as a statement of what occurred in the par- 
ticular season during which this investigation was carried out. 

Light and the evaporating power of the air, 2-week data 

The 2-week graphs of the index values for light and atmospheric evaporat- 
ing power will be treated together since che seasonal marches of these two 
climatic conditions generally exhibit the same main characteristics. Three 
points may be noted in regard to them. (1) Both graphs have, in general, 
a downward slope from the beginning to the end of the season. (2) In the 
majority of cases they agree with each other in direction of slope, from period 
to period, throughout the season. (3) They agree in having a primary maxi- ' 
mum with a very high value, for an early period of the season and one or 
more secondary maxima with lower values, for periods that occur later. The 
secondary maxima of the graphs for light and evaporation sometimes (but 
not always) coincide, as to time of occurrence, with a corresponding maximum 
of the graph for temperature efficiency. The following consideration of the 
individual station graphs for the two conditions may serve to bring out these 
points. 

For Oakland, the primary maximum in the graph of atmospheric evapo- 
rating power (153) occurs in the first period (beginning May 23). The value 
of the evaporation index then decreases steadily to a relative magnitude of 
79, for the first two weeks in July, after which it increases to (104), which 
corresponds in time of occurrence (period beginning July 6) to the secondary 
maximum of the graph of temperature efficiency for this station. After 
passing through this high value the evaporation graph descends again, to 
the low values 57 and 69 for the last two periods (beginning August 27 and 
September 12). The sunshine-intensity index for Oakland varies from an 
initial value of 122 to a final value of 81, with maxima for the periods begin- 
ning July 16 and August 14. Inspection of these two graphs for Oakland 
shows that the direction of slope is the same, from period to period, for the 
greater part of the season. 



Climatic Conditions of Maryland 381 

For Chewsville, the two graphs agree in direction of slope throughout the 
entire season, except between the periods beginning August 25 and September 
8. Both are approximately parallel to the temperature-efficiency graph for 
this station, from the period beginning July 14 to the period beginning August 
25 and both have a downward slope, in general, from the beginning to the 
end of the season. Moreover, they agree in direction of slope from the 
period beginning June 15 to that beginning October 8. 

For College, the evaporation maximum for the period beginning July 27 
corresponds to a secondary minimum in temperature efficiency. The graph 
of the evaporating power of the air for College has a primary maximum for 
the second period (beginning May 28) and a well-marked secondary maximum 
for the period beginning July 22. No sunshine data are available for this 
station. 

For Baltimore, the two graphs in question agree in direction of slope up to 
the period beginning July 9 after which evaporation passes through a second- 
ary maximum which corresponds, in a very rough way, to the double maxi- 
mum of temperature efficiency. 

The Darlington light and evaporation graphs show the general character- 
istics mentioned at the beginning of this discussion, for the greater part of 
the season. The atmometric values for this station are relatively very low, 
all but two of them being less than the seasonal average for all periods and 
stations. 

For Coleman, the sunshine record is incomplete, but the two graphs gen- 
erally agree in direction of slope, so far as comparison is possible, excepting 
between the periods beginning July 17 and July 31. 

For Easton and Princess Anne, the graphs are typical. For the latter 
station, evaporation data are lacking for the periods beginning June 8 and 
June 23. 

The comparatively close agreement between the graphs for sunshine and 
evaporation, for all the stations employed in this study, together with the 
fact that evaporation exhibits no well-defined relation to temperature effi- 
ciency, appears to indicate that the rate at which water evaporated from the 
white cylindrical cups employed as atmometers in this investigation was de- 
termined to a considerable extent by the amount of radiant energy absorbed 
by the cups, and that air temperature played a secondary part in the deter- 
mination of this rate. The fact that the physiological temperature index is 
here used for expressing temperature values does not militate against this 
conclusion, since, as has been previously stated other methods of expressing 
the temperature values give graphs which slope for the most part, in the same 
direction as does the graph of physiological temperature indices. A large 
effect of sunshine on evaporation, the sunshine intensity being measured by 
a black-bulb sunshine recorder, has been found by Briggs and Shantz. 18 



18 Briggs, L. J., and Shantz, H. L. Hourly transpiration rate on clear days as determined by cyclic 
environmental factors. Jour. Agric. Res. 5: 583-650. 1916. 



382 F. Merrill Hildebrandt 

These authors were able to calculate approximately the amount of evaporation 
from a shallow blackened tank using a formula which involved sunshine 
intensity and the saturation deficit of the air, sunshine intensity having a 
preponderating influence. They also state that while the cups and the tank 
respond in different ways to the daily cycle of changes in the evaporating 
power of the air, a certain average ratio exists between the evaporation from 
the tank and that from the cups. It is therefore to be expected from their 
work that the rate of evaporation from Livingston porous cups is largely 
influenced by sunshine intensity, and that air temperature exerts a secondary 
influence on evaporation as measured by these instruments. It must be 
remembered, also, that the evaporation measurements of this study were 
made in the plant enclosures, while air temperature was measured by ther- 
mometers located in a shelter about 1.5 meters (5 feet) above the ground and 
often 4 or 5 meters (15 feet) from the plant enclosures. This may account 
in some measure for the apparent absence of any marked effect of air temper- 
ature on the evaporating power of the air as measured by porous-cup atmome- 
ters. As Livingston has remarked, the porous cups are exposed in some- 
what the same way as are plant leaves, and the foliage of McLean's plants 
was freely exposed to sunshine, as were his atmometers also. Air tempera- 
ture is always obtained from shaded instruments. 

Variability of temperature and evaporation values 

It may be noted that the temperature-efficiency values for the stations 
here considered, exclusive of Oakland, are much more nearly alike for any 
given 2-week period than are the sunshine and evaporation values. The 
values of these three climatic indices for the first 2 weeks of June and for the 
first 2 weeks of August, for the eight stations, are given in table IX. Since 
the dates of observation were not the same for all stations, these values have 
been approximated from the graphs, but they may be considered as suffi- 
ciently accurate to illustrate the manner in which the data at hand support 
the conclusion just stated. 

If the highest value given for each of the three indices and for each of the 
two periods be divided by its lowest value, the ratios presented in the next 
to the last line of the table are obtained. Each ratio represents the magni- 
tude of the range of variation of the climatic index that it represents, for the 
eight stations in question. The average value for these periods is given in 
the last fine. It thus appears that the variation of the temperature-efficiency 
index due to difference in location of the stations is markedly less than is the 
corresponding variation in the index of sunshine or that of evaporation. This 
relation holds generally throughout the season. In short, the temperature- 
efficiency values exhibit a smaller degree of geographical or local variation 
than is exhibited by the index for sunshine or for the evaporating power of 
the air. 



Climatic Conditions of Maryland 



383 



TABLE IX 
Values of the three climatic indices for the first % weeks in June and the first % weeks in 
August, vyith ratio of highest to lowest value for each index, for all stations excepting 
Oakland. 



STATION 


EVAPORATION 


SUNSHINE 


temperature 

efficiency 

(physiological 

index} 




1st 2 

weeks of 

June 


1st 2 
weeks of 
August 


1st 2 

weeks of 
June 


1st 2 
weeks of 
August 


1st 2 

weeks of 

June 


1st 2 
weeks of 
August 


Chewsville 


115 
146 
156 
109 
115 
145 
132 
125 

1.5 


90 
115 
147 
110 

7S 
135 
135 

95 

1.9 


130 
122 

92 
115 
145 
165 
110 

1.8 


95 
103 

77 
112 
120 
115 

75 

1.6 


105 
110 
103 
102 
98 
115 
112 
102 

1.2 


120 


Monrovia 


125 


College 


133 


Baltimore 


152 


Darlington 


125 




152 


Easton 


140 


Princess Anne 


135 


Ratio of highest to lowest value 


1.3 






Average for the 2 periods 


1 


70 


1 


70 


1.25 









Correlation of the 2-week plant and the climatic values 

During the course of this study a number of attempts were made to corre- 
late the climatic measurements with those representing the growth rates 
of the plants, but these were unsuccessful and no scheme applicable in a 
quantitative way to this problem has yet been formulated. For example, 
one of the simpler correlation schemes to be tried was based on the assump- 
tion that the growth of the plants was directly proportional to the index 
values for temperature and light and inversely proportional to those for 
evaporation. Stated as an equation, this assumption takes the form: 



G = 



KTL 

E ' 



in which G represents the. plant growth rate and T, L, and E represent the 
indices of temperature, light and evaporation, respectively, while K is a 
constant of proportionality. Values were obtained for the right-hand mem- 
ber of this equation for the successive 2- week periods for all stations and these 
values were compared with the corresponding growth-rate indices derived 
from the plant measurements. No close correspondence was generally to be 
detected. The equation is given as an illustration of the kind of methods by 
which the discovery of correlations between the plants and their climatic 
environment was attempted. Many combinations of the three climatic 



384 F. Merrill Hildebrandt 

conditions were made and compared with the plant growth rates but, as 
noted above, without satisfactory results. 

It seems probable that the difficulty experienced by every student who has 
thus far attempted this sort of correlation may arise partly from the fact 
that the environmental conditions have not been measured in the right 
way, and partly from the use of inadequate methods for the integration of 
the quantitative data that are obtained. It is hardly to be expected that 
either of the growth criteria here used should be as simple a function of the 
climatic conditions as the formula given above might suggest. Just as soon 
as facilities become available for actual experiments in this field, — experi- 
ments in which all the influential conditions may be controlled and analytic- 
ally understood, — the problem here brought forward prematurely maybe 
seriously attacked. Until such experiments may be begun, all discussion 
regarding the relations between plant growth rates and environmental con- 
ditions must remain vague and unsatisfactory. 

There is no doubt that the distribution of high and low values of any one 
of the climatic conditions, during the growth period of the plants, is an impor- 
tant factor in determining the degree of their development. To take an ex- 
treme case as an example, a few days with a very low sunshine intensity 
would have no direct influence on plants not yet above ground, but such an 
occurrence would exert a very marked influence on plants with a considerable 
leaf area. Obviously, two periods showing similar average values of any 
climatic condition may have a widely differing distribution of high and low 
values of this condition. In the present study, while the distribution of 
high and low temperature and fight values is known, the corresponding 
stages of the development of the plants are not, and it is thus practically 
impossible to take account of this distribution factor. The difficulty of 
correlating growth rates and climatic conditions is further increased by the 
fact that, in measuring dry weight, stem height, etc., we are not measuring 
single processes in the plants, but rather the combined effects of a number of 
processes taken together. 

It is of interest to call attention at this point to certain features of the 
growth of the soy-bean plants of this study whose causes can only be sur- 
mised. These features may be of significance, however, since they show a 
departure from what may be termed the "normal" for plant behavior. In 
the first place, although all of the temperature graphs show two maxima, the 
plants, except in the case of Oakland, failed to respond to the second tem- 
perature maximum by a correspondingly high rate of growth. For Chews- 
ville, as an example, in the period beginning August 11, we have a low value 
for the leaf-product with a high temperature index and the other conditions 
at about the seasonal average. For Monrovia in the period beginning August 
10, with a high temperature value and with sunshine intensity at about the 
seasonal average, the plants show a relatively low value of the leaf-product. 



Climatic Conditions of Maryland 385 

This may be contrasted with the period beginning June 15 for this same sta- 
tion, which, with a leaf-product about the same as that of the first-mentioned 
period, seems to show less favorable growing conditions — namely, a much 
lower relative temperature index, a very high evaporation rate and a sun- 
shine value only a little higher than the corresponding value for the period 
beginning August 10. For College, the periods beginning July 17, July 31 
and August 14, with about the same values for temperature and evaporation, 
show magnitudes of 152, 204 and 150 respectively for the leaf-product. This 
variation may possibly be related to differences in the value of sunshine inten- 
sity for these periods, but sunshine data are lacking for this station. For 
Baltimore, the periods beginning July 23, August 6 and August 20 show large 
differences in the leaf-product with comparatively slight differences in the 
climatic conditions. Evaporation was slightly less rapid for the period 
beginning July 23 than for the period beginning August 8, and considerably 
less for the period beginning August 20, but this seems to have occurred with- 
out the expected effect on the plants. For Coleman the plant graph slopes 
upward to a value of over 200 for the period beginning July 8, while for the 
period beginning August 5, which has climatic conditions apparently as favor- 
able, the relative value of the leaf -product is only 138. For Easton the 
leaf-product is lower than would be expected for the period beginning August 
17, and for Princess Anne the plant values for the period beginning August 8 
are much lower than for the period beginning July 7, which had approxi- 
mately the same climatic conditions as the first-mentioned period. 

A second feature of the plant graphs, and one that cannot be correlated 
with the climatic data, is that the rate of stem elongation reaches its highest 
value for the season before the occurrence of the maximum leaf-product, for 
all stations except Darlington and Coleman. For Darlington, the highest 
value for stem height and leaf-product both occur for the period beginning 
July 10 and for Coleman the maximum value for stem height occurs for the 
period beginning July 22, while the leaf-product reaches its highest value for 
the season at this station in the preceding period. For the remaining sta- 
tions, the highest value for stem height occurs two weeks or a month earlier 
than does the highest value for leaf-product. 

THE 4-WEEK VALUES 

The 4-week plant and climatic data derived from the exposed stations are 
presented in the tables and graphs already explained, and the following con- 
sideration of these values will refer to the graphs, as in the case of the 2-week 
values. 



386 F. Merrill Hildebrandt 

The 4-week plant data for stations in the open (see fig. 4, black lines) 

For the 4-week data, the rate of stem elongation may be compared with 
the rate of leaf expansion as determined from actual measurements of leaf 
area. This comparison shows, the same general relations as appeared to exist 
between stem height and leaf-product for the 2-week growth periods. Owing 
to the fact that the 4-week plants were grown for a longer time, however, the 
4-week data show fewer cases with the rate of stem elongation greater than 
the rate of leaf expansion. In most cases the rate of stem elongation is con- 
siderably smaller than the rate of leaf expansion. This illustrates the tend- 
ency of the soy-beans to show a low rate of height growth relative to the 
rate of leaf expansion when both rates are large. 

For Oakland, the stem-height graph is above the leaf area graph for the 
first three periods of the season and below it for the other five periods. For 
Chewsville the three plant graphs follow each other very closely and the 
differences in their relative positions are probably due, for the most part, to 
individual variations in the plants of the separate cultures. The Monrovia 
graphs also support the assumption that stem height shows a well-defined 
tendency to remain below leaf area during the first part of the season. For 
College, the stem-height graph is below that for leaf area for the entire season, 
except for the two periods beginning June 19 and September 25. The Bal- 
timore graphs show stem-height values higher than the corresponding leaf- 
area values for the periods beginning May 14, May 29, June 10 and August 
20, due possibly to low light intensities. The Darlington cultures show very 
high values of both growth rates, with stem height below leaf area for the 
entire season. For Coleman, the stem-height graph remains below the leaf- 
area graph for all the periods except the last, in which case it rises very slightly 
above the leaf-area graph. For Easton, the two growth rates are about 
alike, showing nearly the same relative values for each culture period. 
For Princess Anne, the stem-height and leaf-area graphs show a departure 
from the usual behavior during the first three periods of the season. For 
these periods, the leaf-area values are relatively large and those for stem 
height are relatively low, for some reason not apparent from the climatic 
conditions. 

A very striking relation is shown between the 4-week values for leaf area 
and for dry weight. For most of the cultures these two kinds of growth 
rates have practically the same relative numerical values for any given period. 
The Oakland graphs show this for all periods except the one beginning June 
5, for which dry weight is markedly larger than leaf area. For Chewsville, 
the general relation just mentioned shows very well throughout the season. 
For Monrovia, the leaf-area value shows a rather large deviation from the dry- 
weight value for the periods beginning June 16, June 30 and Aug. 25, but 
otherwise the two growth rates correspond in their relative values during the 



Climatic Conditions of Maryland 



387 





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rv\i hai June ji>f<i jui-i jiii ajji; au- au* sepi 5 






Fig. 4. Graphs of 4-week data for exposed stations as named (continued). 
(Lines as in 4- week graphs of fig. 3.) 



388 F. Merrill Hildebrandt 

entire season. The College graphs show close agreement, with diy weight 
above leaf area during the first part of the season. For Baltimore, the rela- 
tive leaf-area value differs considerably from the value for relative dry 
weight for the periods beginning August 6 and October 1, but the remaining 
periods show close agreement. The two Darlington graphs show close agree- 
ment for all periods. For Coleman, dry weight and leaf area agree well for 
all periods, except those beginning August 5 and August 19. For Easton, 
no large differences between these two rates occur for any of the cultures. 
For Princess Anne, the period beginning August 4 is the only one showing a 
difference of considerable magnitude between the relative leaf-area value and 
the dry-weight value. 

This property or characteristic of soy-bean renders possible the use of the 
leaf area of the plant as an index of the dry weight of the tops, and appears 
to render soy-bean particularly promising as a standard plant for climatic 
investigations, as has been pointed out in a previous paper,' 9 from which the 
following paragraph is taken. 

"If the method proposed by Livingston and McLean (1916), of employing 
the growth rates of standard plants as indices for the comparison of different 
climates as these influence plant growth in general, is to be of value, it is of 
course necessary that suitable plant characteristics be chosen for measure- 
ment in determining the growth rates, and it is desirable that the measure- 
ments be such as may be made from time to time without injury to the 
plants. The most generally accepted criterion of plant growth, dry weight 
of tops, can be obtained but once for any individual plant, since the plant is 
destroyed during the determination. Also, the accurate determination of 
leaf area is very difficult unless the plants are destroyed. On the other hand, 
as McLean has emphasized, leaf dimensions may be obtained repeatedly 
during the development of the plant, without serious danger of inflicting 
injury. It may therefore be of considerable importance if leaf area, and even 
dry weight can be satisfactorily estimated for soy-bean by the employment 
of the leaf-product as an index." 

Dry weight and actual leaf area were both determined only for the 4-week 
periods, the plants being then destroyed, but the lengths and breadths of 
all leaflets were obtained for both the 2-week and the 4-week periods. Con- 
sequently, to study the correlation between total leaf area and total ieaf- 
product per plant, only the 4-week data are available and these are the 
ones here considered. 

Since soy-bean leaflets are approximately elliptical in form and since the 
area of an ellipse is proportional to the product of its axes, the leaflet-product 
(length times breadth) of any leaflet should be nearly proportional to the 
area of that leaflet. Whether this relation may hold during the growth of 

19 Hildebrandt, F. M. Leaf product as an index of growth in soy-bean. Johns Hopkins Univ. Circ, 
March, 1917. P. 202-205. 



Climatic Conditions of Maryland 389 

the leaflet under different sets of climatic conditions depends upon how nearly 
the elliptical form is retained. The sum of the individual leaflet-products of 
any plant, which is the total leaf-product for that plant, should be approxi- 
mately proportional to the total leaf area of the plant, if the relation given 
above holds. In the discussion that follows it will be shown that such an 
approximate proportionality does exist in the case of the 4-week soy-bean 
plants. 

In order to find out whether the actual areas of the leaves in these cultures 
were proportional to the corresponding leaf-products, the ratio of the two 
quantities was worked out for a number of the stations. It was found that 
the leaf-product divided by the leaf area gives a number that varies only 
slightly from the value 1.28. In other words, if we measure the two diame- 
ters of the leaflets of a 4-week soy-bean plant, multiply these two numbers 
for each leaflet, and add the products, a number is obtained which, when 
divided by 1.28, closely approximates the actual leaf area of that plant. 
Instead of using the sum of the products of length and breadth as an index 
of the area per plant we may use the sum of the squares of the lengths of the 
leaflets or the sum of the squares of the breadths of the leaflets. The numbers 
thus secured do not, however, bear as nearly constant a ratio to the actual 
leaf area as does the total leaf-product, and hence neither is as satisfactory 
an index of the area as is the leaf-product itself. 

One of the most interesting properties of the 4-week soy bean plant is that 
the dry weight of stem and leaves is approximately proportional to the total 
leaf area. Having, therefore, a means by which the leaf area may be con- 
veniently estimated, it is possible to calculate the dry weight of the plant 
approximately, by multiplying the leaf-area by the proper constant. The 
proportionality between the weight of the plant and its leaf area is not quite 
so constant as that between leaf area and leaf-product, but in the great 
majority of cases the variation in the ratio of dry weight to leaf area, from 
a constant value, is less than 10 per cent. The relations given hold over 
a very wide range of climatic conditions and for plants varying in height 
from 2 or 3 cm. to 18 or 20 cm. Since none of the plants in these experi- 
ments were grown to maturity, it is impossible to say whether this relation 
holds up to that time. 

From the foregoing facts it may be concluded that the dry weight and 
leaf area of soy-beans 4 weeks old from the seed can be determined approxi- 
mately from their leaflet dimensions. Soy-bean should therefore be very 
suitable for use as a standard plant for the measurement of climate in the 
manner suggested by Livingston and McLean, since the rate of its growth 
can be approximately determined from easily obtained leaf measurements. 
Also, the properties of soy-bean given above should make it a useful plant 
for any piece of physiological research in which it is desired to know approxi- 
mately the dry weight of the plant used, at various stages of its development. 



390 F. Merrill Hildebrandt 

The 4-week climatic data for stations in the open (see fig. 4> fed lines) 

It will be remembered that the cultures were started every two weeks and 
that each grew for a period of four weeks. The 4-week periods thus over- 
lap, and attention has been called to the fact that averages of the climatic 
factors for these over-lapping periods form a smoother graph than averages 
for the 2-week periods. The 4-week graphs, therefore, show the general 
seasonal march of the index values for various stations better than do the 
2-week ones, while the latter show the details of the seasonal march better 
than the former. This fact will be brought out by a brief reference to the 
graphs at this point. 

The values of the physiological temperature indices for the 4-week periods 
show the seasonal marches of this condition for the various stations, from 
low values in May to high midsummer values, and then to low values again 
in the last part of the season. The graphs for all of the stations except 
Oakland show a steeper slope after the midsummer maximum has been 
passed than for the periods during which the temperature was rising to this 
maximum. The two maxima that were present in most of the 2-week graphs 
are eliminated in the 4-week averages and the graphs of temperature values 
show instead a period of about 6 weeks during which this condition remains 
approximately constant. 

The 4-week evaporation and light data show the general characteristics of 
the seasonal marches of these conditions previously noted as exhibited by the 
2-week data. It will be seen, in the first place, that both graphs exhibit a 
downward slope from the beginning to the end of the season; and, in the 
second place, that both graphs show, in addition to their high primary maxi- 
mum in the early part of the season, one or more secondary maxima later. 
In some cases the secondary maxima of the evaporation graphs coincide, as to 
time of occurrence, with temperature maxima. Both of these general char- 
acteristics shown in the 4-week graphs of evaporation and light are shown by 
the 2-week graphs but since small variations are ehminated by averaging 
the over-lapping periods, there are fewer secondary maxima in the 4-week 
graphs. In the case of evaporation, there is usually one secondary maximum 
occurring in or near the 4-week period including the last 2 weeks of July and 
the first 2 weeks of August. In the case of all stations this is one of the 
three 4-week periods showing high temperature values. The 4-week climatic 
graphs need not be discussed further here. The method by which the 4-week 
data were derived from the 2-week data amounts to the same thing as smooth- 
ing the 2-week graphs and only the more pronounced characteristics of the 
graphs remain after averaging. Interest in the 4-week climatic data thus 
lies mainly in their relation to the plant growth rates. 



Climatic Conditions of Maryland 391 

Results for the Three Covered Stations 
introductory 

All of the data discussed up to this point were obtained for the open, with 
no covering other than a screen of wire netting of large mesh, to protect the 
plants from injury. At three of the stations, Oakland, Baltimore and Easton, 
as has been noted, a series of cultures was also grown under glazed cold-frame 
sash, supported three feet above the ground, these cultures being designated 
as the Oakland, Baltimore and Easton covered stations. The behavior of 
the plants grown under glass was very different from the behavior of those 
grown in the open, and the results for the covered stations will be considered 
in this section. 

The covered cultures were placed near the exposed cultures at each of the 
three places mentioned, so that the climatic conditions for the two would be 
practically the same, except as modified by the glass. 

THE PLANT DATA, COVERED STATIONS 

(See figs. 5 and 6, black lines) 

The effect of the glass cover was shown by the plants in two ways: (1) 
growth was always greater for the covered stations than for the exposed, 
and (2) the plants of the covered stations showed a marked difference in 
manner of growth from the plants of the exposed stations. The greater 
growth of the covered plants was shown in some cases by one, in some cases 
by two, or even by all three of the growth measurements taken. Not only 
did the plants show greater growth, but the maxima in the graphs of the 
various growth measurements for the covered plants do not usually occur at 
the same times as do the maxima in the corresponding graphs for the plants 
grown in the open. The principal effect of the covering on the way in which 
the plants grew is shown by a disturbance of the relation between dry weight 
and leaf area. In previous discussion of this relation for the exposed plants 
it was noted that the relative dry-weight and leaf-area values are approxi- 
mately the same for the 4-week plants. In the case of the covered stations, 
on the other hand, every culture shows relative leaf area as higher (usually 
very much higher) than relative dry weight. Stem height for the covered 
cultures usually shows high .values as compared to the corresponding exposed 
cultures. The tendency noted in previous discussion for this growth rate to 
fall off relatively, as the plants become larger, seems to be only slightly in 
evidence here. The following consideration of the covered cultures in detail 
will bring out these features. It should be noted that the culture periods for 
the covered stations each agree in length, to within a day or two, with those 
for the corresponding exposed stations. Such slight differences as exist in 



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Fig. 5. Graphs of 2-week and 4-week data for covered and forest 
stations, as named. 



Black, as in figs. 1 and 3. 
Red, Evaporation index. 



392 



Climatic Conditions of Maryland 



393 



the lengths of the culture periods do not in any degree account for the differ- 
ences in the plant measurements nor interfere with the general comparisons 
here made. In comparing growth for the exposed and covered cultures, no 
attempt will be made to account in detail for the differences between the 
two sets of plants in terms of climatic conditions, since the climatic influencs 
acting on the covered plants are not even so well known as in the case of the 
exposed stations, and it has already become clear that a really satisfactory 
interpretation of growth rates by means of such climatic measurements as 
are here employed is nearly hopeless at present. After the peculiarities of 
the covered plants have been pointed out, however, some suggestions as to 
the probable causes of these peculiarities will be brought forward. 



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Fig. 6. Graphs of 2-week and 4-week data for Easton covered station. 
(Lines as in fig. 5.) 

The Oakland covered station. — The covered and exposed cultures for Oak- 
land differ less than do the corresponding sets for Baltimore and Easton, but 
they show the general features outlined above. The plants of the 2-week 
covered cultures for Oakland exhibit a much higher value of the leaf-product 
than do the corresponding exposed cultures, for the periods beginning June 
18, July 2, and July 15, and the stem-height value is greater for the covered 
station than for the exposed station, for the periods beginning June 4, July 2 
and July 15. The highest value of leaf-product occurs for the period begin- 
ning July 15 for the covered, and in the period beginning July 16 for the 



394 F. Merrill Hildebrandt 

exposed 2-week plants. Each set of cultures show two seasonal maxima in 
the plant graphs, but these are much higher in the case of the covered plants 
than in the case of the exposed. In the 4-week graphs for the covered plants, 
leaf area is higher than dry weight for the whole season, while the exposed- 
station graph for leaf area is well below that for dry weight, from the period 
beginning May 23 to the period beginning July 16, inclusive. The maximum 
for all the growth measurements of the 4-week exposed plants occurs for the 
period beginning June 19, while the maximum for the covered station occurs 
for the period beginning July 2. Also, the graphs for the 4-week plants all 
exhibit higher values than do the graphs for the 2-week plants for most of the 
culture periods of the season. This is especially true of leaf area. Covering 
the plants with glass seems to have produced a relatively high rate of leaf 
expansion, in spite of the fact that the evaporation value is somewhat higher 
for the covered than for the exposed plants. 

The Baltimore covered station. — The 2-week plant data for the covered sta- 
tion at Baltimore are plotted to a scale one-half as great as the scale used in 
plotting the exposed plant values, on account of the high values of stem height 
and leaf-product shown by the covered culture beginning July 9. The values 
of both leaf-product and stem height for this station are both uniformly 
above the corresponding values for the exposed station. Also, the tendency 
of the covered plants to elongate relatively more rapidly than did the exposed 
plants is shown by the stem-height values for Baltimore covered station for 
both the 2- and 4-week periods. It is interesting to note that the covered 
plants do not show specially high values of the plant growth rates for the 
period beginning August 6, as do the exposed plants. The 4-week graphs for 
the covered plants show very well the tendency of leaf area to reach values 
relatively higher than those for dry weight, the leaf-area graph being well 
above the dry-weight graph for the entire season. 

The Easton covered station. — For Easton the covered plants, as compared 
with the exposed, show the general tendencies noted above. The 2-week 
growth rates of the covered plants, especially for stem height, are higher than 
the corresponding rates of the exposed plants. It will be observed that the 
maximum growth for the season, in both the covered and exposed 2-week 
cultures occurs for the period beginning August 3. The 4-week plants of 
the covered cultures show leaf area relatively higher than dry weight. The 
values for the culture period beginning June 22 are relatively low for the 
covered as well as for the exposed cultures. 

THE CLIMATIC CONDITIONS, COVERED STATIONS 

(See figs. 5 and 6, red lines) 

Of the three climatic factors generally dealt with in this study, evaporation 
alone was measured for the covered stations, so that the climatic data are 
much less satisfactory in this case than in the case of the exposed stations. 



Climatic Conditions of Maryland 395 

It is safe to suppose that the climatic conditions under the glass differed from 
those for the corresponding exposed stations in certain definite ways, period 
by period. The rate of evaporation for the covered stations was considerably 
greater than for the exposed as will be seen by comparing the values given in 
the tables. We may be certain, also, that some of the incident light was 
absorbed by the glass and that the light intensity under the cover was thus 
less than the intensity of the light falling on the exposed plants. Also, we 
may be reasonably sure that the air temperature under the glass was some- 
what higher than that outside, especially on quiet days when circulation of 
air was slight, and there was little tendency toward equalization of air tem- 
peratures. In considering the behavior of the covered cultures as related to 
climatic conditions, it may be mentioned that evaporation is known to have 
been more intense and light intensity lower for these than for the corre- 
sponding exposed stations and periods, while air temperature was probably 
higher for the covered than for the corresponding exposed stations. 

The differences between the behavior of the plants under glass and that of 
the plants in the open seems to be primarily attributable to differences in 
light conditions for the two sets of cultures. The more rapid stem elongation 
occurring under glass is exactly what would be expected if the air temperature 
was higher and the light intensity was lower than in the case of the corre- 
sponding exposed cultures. The fact that leaf area is relatively high for the 
covered plants, as compared with their final dry weight, may possibly be 
related to a smaller amount of dry matter produced by photosynthesis per 
unit of leaf area in the covered cultures. Such a difference might be expected 
if the light energy available for photosynthesis were cut down by interposing 
between the plant and the light source a screen that absorbed a part of the 
light. 

Whatever may be the true explanation of the behavior of these plants 
under glass (and the true explanation will surely be much more complicated 
than is here suggested), the facts indicate very clearly that the growth of 
the plants under glass was quite different from the corresponding growth 
in the open. This point must be important in physiological experiments con- 
ducted in greenhouses. 

Results foe the Baltimore Forest Station 

(-See fig. 5) 

The Baltimore Forest Station was located about 150 yards from the 
exposed and covered stations at that place. Evaporation was the only 
climatic feature measured for this station. The sunshine intensity was of 
course very low, due to the shading and screening effect of the leaves of the 
trees above the experimental plants. Air temperature was also probably 



396 F. Merrill Hildebrandt 

considerably lower than that experienced by the exposed and covered plants. 
The modification of growth habit in the case of the forest plants is very 
striking, as can be seen by an inspection of the plant graph for this station. 
The soy-beans were short erect growers in the open, and were erect with long 
stems under the glass of the covered station, but were runners in the forest. 
This effect on stem growth, which obviously cannot be explained as an effect 
of temperature alone in the case of these cultures, is relatively very great, 
the highest 2-week value for stem elongation being over four and a half 
times as great as the seasonal average for all periods and stations, and the 
highest 4-week value was a little less than four and a half times the seasonal 
average. As compared with plants grown in the open, the 4-week forest 
plants also show the same reversal in the relative positions of the leaf-area 
and dry-weight graphs as was shown by the covered plants. The leaf-area 
graph is above the dry-weight graph for the entire season in the forest. 
These cultures are thus more like the covered ones than they are like the 
exposed ones. This may possibly be accounted for by supposing that the 
similarity in the behavior of the plants in the covered and forest stations at 
Baltimore was related to a corresponding similarity in the light conditions for 
these two sets of cultures, but the problem is doubtless very complex. 

THE PLANT DATA AS MEASURES OF THE CLIMATIC 

EFFICIENCY FOR GROWTH OF THE 

STANDARD PLANTS 

INTRODUCTORY 

As has been stated, the investigation of which this study is a part was 
planned with the idea of obtaining some quantitative measures of the cli- 
matic complex for each of the various stations, in terms of plant activity. 
Since the soil used was the same, since its moisture content was kept high 
enough to support good growth at all times, for all stations and for all peri- 
ods, and since seeds of the same lot were used in all cases, it is supposed that 
the differences in the growth rates for the various periods and stations must 
have been due to effective environmental differences other than those of soil 
conditions. On account of the auto-irrigation of the cultures, precipitation 
was practically without direct influence upon the cultures of the exposed and 
forest stations, and it was of course quite without direct influence upon the 
cultures of the covered stations. The influential environmental conditions 
that differed from station to station and from period to period in these tests 
were those usually considered as climatic, with the omission of precipitation. 
The plant data, as set forth in the tables and graphs, may therefore be re- 
garded as approximate measures of the integrated non-precipitation condi- 
tions of the several climatic complexes under which the plants grew. These 



Climatic Conditions of Maryland 397 

measures of course refer specifically to this particular variety of soy-bean 
plant and to the particular set of soil conditions that was common to all cul- 
tures. With another soil, or with another kind of plant, the plant values 
would of course have been more or less different from those here recorded. 
It remains to be found out whether or not soy-bean is a suitable standard 
plant for use in this sort of climatic integration when the needs of agriculture, 
forestry and general ecology are primarily considered. From what has been 
said in the preceding sections it appears, however, that soy-bean is at least 
especially well suited to preliminary and pioneer studies like the present one. 20 

From this point of view, each of the graphs of the plant values (shown by 
the black lines in figures 2-6) may be regarded as a representation of the sea- 
sonal march of the non-precipitation portion of the climatic environment 
for the particular station in question, the graphs for the exposed stations rep- 
resenting the "natural" conditions, while those for the covered and forest 
stations refer to the more or less modified climates experienced by these 
cultures. Some of the more outstanding features of these plant graphs have 
been mentioned in the preceding sections of this paper, and other features 
will become evident from a careful study of the graphs themselves, or of the 
tables from whose data the graphs were constructed. Much more might be 
said in this connection than has been said, but the newness of the present 
point of view, together wth the obvious complexity of the numerical results 
here presented, make it undesirable to attempt a careful study of these data 
at the present time. The tables and graphs of this paper render the numer- 
ical values available for future study, when this aspect of climatology and 
ecology shall have begun to attract more general and appreciative attention 
than it now enjoys. 

It should be emphasized that the plants have automatically weighted and 
integrated all the fluctuating and differing conditions for the several culture 
or exposure periods, and that the final summation is given in terms of the 
amount of growth produced in 2 weeks or 4 weeks from the seed. Dividing 
this final summation by the number of days in the corresponding period 
gives the average plant producing power of the non-precipitation part of the 
climatic complex for the given period and station. 

It has been noted that these plant values generally show a seasonal march 
for each station, the growth index being relatively low for periods near the 
beginning and end of the season, and relatively high for midsummer periods, 
and it has been suggested that temperature may be considered as the main 
controlling condition in the bringing about of these seasonal marches, various 
modifications being superimposed upon the temperature influence by other 
climatic conditions such as the intensity, duration and seasonal distribution 
of light, and the intensity and seasonal distribution of evaporation. 



20 A study somewhat similar to this one, using wheat, pea and Drome-grass as standard plants, was carried 
out by Sampson assisted by the author. See: Sampson, A. W. Climate and plant growth in certain vege- 
tative associations. U. S. Dept. Agric. Bull. 700. 72 p., 37 fig. Govt. Printing Office: Washington, 1918. 



398 



F. Merrill Hildebrandt 



SEASONAL AVERAGES OF MEAN DAILY INTENSITY VALUES FOR THE SEVERAL 

STATIONS 

Aside from the characteristics of the seasonal marches of the climatic con- 
ditions in question (which are best seen in the seasonal graphs themselves, 
figs. 2-6), it is of interest to average all the corresponding plant-index values 
for the season for each station, thus obtaining a seasonal average or mean 
daily plant -producing power, as a single index for each growth criterion for 
each station. This has been done for all the stations, for the 2-week and for 
the 4-week periods and for each growth criterion, and the resulting seasonal 

TABLE X 

Relative seasonal daily means for the several stations, by each of the five growth criteria. 
The letter H denotes high values; M, intermediate values; and L, low values. (The 
covered station and the forest station are included for completeness.) 



STATION NAME 



Oakland 

Chewsville 

Monrovia 

College 

Baltimore 

Darlington 

Coleman 

Easton 

Princess Anne. 



Oakland, covered. . 
Baltimore, covered. 
Easton, covered 



Baltimore, forest. 



NUMBER 


2-WEEK 


2-WEEK 


4-WEEK 


4-WEEK 


OF 


STEM 


LEAP 


STEM 


LEAF 


DAYS 


HEIGHT 


PRODUCT 


HEIGHT 


AREA 


125 


L86 


L77 


L71 


L71 


154 


L87 


L84 


L75 


L74 


154 


L79 


L80 


L66 


L71 


154 


M95 


M101 


M80 


mho 


153 


H125 


H119 


H104 


Ml 15 


154 


H113 


H118 


H106 


H148 


168 


M96 


M107 


M80 


M117 


171 


M95 


M105 


M75 


L82 


169 


M106 


M96 


M92 


M117 


— 


M92 


L78 


M81 


M118 


— 


H172 


H125 


H136 


H154 


— 


H145 


H142 


HI 16 


H137 


— 


HIP271 


LL a 62 


HH^SG 


L69 



4-WEEK 

DRY 
WEIGHT 

L79 

L78 

L79 

Ml 19 

M105 

H144 

Ml 13 

L83 

M116 

M93 

M95 

M109 

LL a 41 



a The doubling of a letter indicates an extreme condition; HH means very high, etc. 

means are shown in the last column of each of the data tables (tables I— 
VIII), where the corresponding seasonal averages for the climatic data are 
also given. It is to be remembered that the values are all relative, each one 
being stated in terms of the corresponding average for all stations and all 
periods, this unit being considered as 100. 

The seasonal averages for the various exposed stations are brought together 
in table X and are shown graphically in figures 7 and 8, the former figure 
dealing with the 2-week and the latter with the 4-week plant data. The 
abscissas of these graphs are not quantitative; the vertical lines are equally 
spaced and each one represents one of the exposed stations. The stations are 
arranged in the order of their geographical locations, as far as this is possible 
in a linear series. The ordinates of these graphs represent the seasonal 
means. 



Climatic Conditions of Maryland 



399 







400 F. Merrill Hildebrandt 

The 2-week seasonal averages of the two growth measurements taken for 
the nine exposed stations, represented graphically in figure 7, show that the 
plant-producing power of the climatic complex is about the same whether it 
is measured by stem height or leaf-product. The range of variation for stem 
elongation is from 79 (Monrovia) to 125 (Baltimore). In terms of this 
growth measurement, the average intensity of the Monrovia climatic com- 
plex is 63 per cent of that of the corresponding Baltimore complex. Similarly, 
the leaf-product mean varies from a minimum of 77 (Oakland) to a maxi- 
mum of 119 (Baltimore); as measured by leaf -product, the mean intensity 
of the Oakland climate is 65 per cent as efficient as the corresponding mean 
for the Baltimore climate. Precipitation is of course left out of account here, 
as in the other considerations of this paper. The nine stations fall into 
three groups, according to these mean values: Oakland, Chewsville and 
Monrovia have low relative values, Baltimore and Darlington have high 
values, and College', Coleman, Easton and Princess Anne have intermediate 
and similar values. (See the letters L, H and M in table X.) 

Turning to the 4-week seasonal averages, as shown in figure 8, it is seen 
that the graph for stem height agrees very well with the two 2- week graphs 
just considered. It is also seen that the 4-week graphs for leaf area and dry 
weight agree in a satisfactory manner. According to these two graphs, the 
nine stations fall into the following three groups: Oakland, Chewsville, Mon- 
rovia and Easton constitute the groups with low values, Darlington is alone 
in the group with high values, and College, Baltimore, Coleman and Princess 
Anne make up the group with intermediate and similar values. (See the 
letters of table X.) 

It is to be remembered that the two series of data (2-week and 4-week) 
refer to the same total time interval. The plants of one series were register- 
ing the same climatic conditions as those of the other series; indeed, they 
were the same plants, for the 4-week measurements were obtained from the 
same plants as those from which the corresponding 2-week measurements 
had been secured. The fact that the seasonal averages of the leaf area 
values (4-week) do not show the same grouping of the stations as do the leaf- 
product values (2-week) is to be referred to the fact that the plant alters its 
internal conditions with growth and age. A soy-bean plant exposed two 
weeks is an entirely different instrument (as far as measuring environmental 
efficiency is concerned) from the same plant exposed 4 weeks. For this 
reason it seems desirable that such studies as the present one should be car- 
ried out with as short periods of exposure of the standard plants as is feasible. 
It is somewhat as though the instrument wore out and altered its character- 
istics with too long exposure. Since it is obviously impracticable to obtain 
a large number of plants that are approximately alike, excepting as seeds, it 
seems desirable to begin each observation with new seed (as was done in 
this investigation), and to take the final readings before the internal condi- 



Climatic Conditions of Maryland 



401 




402 • F. Merrill Hildebrandt 

tions of the plants have been too seriously altered through age and the 
approach toward maturity. At the same time, the standard plants must of 
course be allowed to grow long enough so as to be influenced by the fluctuat- 
ing environmental conditions and long enough to give easily-obtained meas- 
urements. McLean (1917) has given some attention to the difference between 
the behavior of the soy-bean plant during the first and second two weeks of 
its growth from the seed, under the same set of climatic conditions and 
fluctuations, pointing out that the plant becomes more sensitive to evapora- 
tion conditions as it grows older (since its leaf surface becomes larger). In 
the use of standard plants as indicators of climatic efficiency the length of 
time chosen for the exposure period is clearly very important. It may be 
added that future studies may bring out certain advantages for a 3-week or 
4-week exposure of soy-bean plants, as compared with a 2-week exposure, 
but — as has been pointed out elsewhere in this paper — details will be more 
apparent when the periods are relatively short, and the principles upon which 
this sort of work is based are more nearly fulfilled with short periods. 

To summarize this discussion, the nine exposed stations arrange themselves 
in three groups by every one of the five criteria, the grouping is identical by 
three of the criteria (2-week stem height, 2-week leaf-product and 4-week 
stem height), it is identical by the two remaining criteria (4-week leaf area 
and 4-week dry weight), but is it somewhat different by these two separate 
■ series of criteria. The differences are : that the second series of criteria 
place Baltimore in the intermediate instead of in the high group, and Easton 
in the low instead of in the intermediate group. 

It is a striking fact that all five growth criteria agree in placing Oakland, 
Chewsville and Monrovia in the group for low mean daily values, in giving 
Darlington high values, and in giving College, Coleman and Princess Anne 
intermediate values. Only for Baltimore and Easton, among the exposed 
stations, are there discrepancies. 

If the five seasonal values are averaged for each exposed station, the result 
places Oakland (77), Chewsville (80) and Monrovia (75) in the group fol- 
low averages, gives intermediate values for College (101), Coleman (103), 
Easton (88) and Princess Anne (105), and gives high values for Baltimore 
(114) and Darlington (126). These average values are shown, in the third 
column of table XI. 

The average data for the covered and forest stations, also shown in table 
X, emphasize the influence of the glass covers and of the forest shade, etc. 

It is perhaps important to emphasize that the criterion of stem elongation 
gives the same grouping of the exposed stations by the 4-week as by the 
2-week values. The ratio of the 2-week seasonal mean to the corresponding 
4-week mean is shown for each exposed station below. 



Climatic Conditions of Maryland 



403 



Oakland 1.21 

Chewsville 1.16 

Monrovia 1 . IS 

College 1.19 

Baltimore 1 .20 



Darlington 1 .07 

Coleman 1.20 

Easton 1.27 

Princess Anne 1.15 



The average of these ratios is 1.18. If, therefore, the stem height of the 
soy-bean, grown as a standard plant, be used as a measure of the climatic 
complex, and the measurement be expressed as relative average daily incre- 
ments, as in this study, the 2-week readings may be approximately reduced 
to 4-week readings (considering these as the standard) by dividing each 
2-week reading by the constant 1.18. 



THE TOTAL SEASONAL EFFICIENCIES FOR THE SEVERAL STATIONS 

The efficiency of an environmental complex, or its power to produce growth 
in a standard plant, is to be considered as the product of two factors, intensity 
and duration. If the seasonal averages of the mean daily rates of growth, 

TABLE XI 
Relative generalized climatic-efficiency products for the several stations. 



STATION NAME 



Oakland 

Chewsville 

Monrovia 

College 

Baltimore 

Darlington. . . 

Coleman 

Easton 

Princess Anne 





RELATIVE 




NORMAL LENGTH OF 


GENERALIZED SEA- 


CLIMATIC 


GROWING SEASON 


SONAL AVERAGE 


EFFICIENCY PROD- 


(A) 


OF DAILY MEAN 
INTENSITY (B) 


UCT (ab) 


days 






117 


77 


9009 


156 


80 


12480 


— 


75 


— 


167 


101 


16867 


223 


114 


25422 


188 


126 


23688 


205 


103 


21115 


201 


88 


17688 


181 


105 


19005 



to which attention has thus far been confined, be taken as the intensuy fac- 
tors for the respective stations, for the season of 1914, and if the length of 
the entire growing season for each station be taken as the (duration factor, 
or the length of time through which the corresponding intensity is consid- 
ered as effective, then the product of the length of the season and the corre- 
sponding intensity factor should give a value that may approximately repre- 
sent the relative efficiency of the climatic complex for the station and year 
in question, by the given plant criterion. Precipitation is of course neglected, 
as it was not involved in this study. 

The seasonal averages of the daily means for the growth rates, as used in 
this study (table X), may be taken to represent the relative values of the 



404 F. Merrill Hildebrandt 

climatic intensities dealt with, but the lengths of the growing seasons are 
only approximated by the total lengths of the test periods. Rather than to 
employ these lengths it will perhaps be better to use the mean (normal) 
lengths of the growing seasons for the several stations here considered. 
These may be obtained from Fassig's paper on this subject, 21 and they are 
shown in table XI, along with the corresponding generalized climatic efficiency 
products, obtained by multiplying Fassig's mean length of the growing season 
by the corresponding average climatic intensity (including all five plant cri- 
teria) as developed in the preceding section of this paper. It is to be empha- 
sized that the intensity factors are all for the summer of 1914 and that the 
duration factors are normal, or at least closely approximate normal values. 

From table XI it appears that the lowest efficiency product is for Oakland, 
as would be expected, while the highest is for Baltimore. The Baltimore 
value is nearly thrice as great as is the value for Oaldand. If we regard 
values above 20,000 as high and those between 10,000 and 20,000 as inter- 
mediate, the stations may be grouped as follows: — 

Low values: Oakland. 

Intermediate values: Chewsville, College, Easton and Princess Anne. 

High values: Baltimore, Darlington and Coleman. 
These efficiency products may be taken to represent, more or less approxi- 
mately, the relative values of the climatic conditions at the various stations, 
to produce plant growth when irrigation is resorted to, so that drought periods 
are avoided as far as soil moisture is concerned. While there is no reason for 
thinking that these values (obtained from 2-week and 4-week periods and soy- 
bean plants, with the particular soil used in this study) may give really 
quantitative information on these climates as related to plant growth in 
general, still the product indices here derived are perhaps more reliable than 
any other series of numerical values that might be readily obtained, and they 
illustrate a new method by which a beginning may be made aiming toward 
the quantitative comparison of climatic complexes. 

One of the aims of ecological climatology should be to evaluate climates 
in somewhat the same manner as water-power, mineral deposits, and other 
geographically restricted sources of power for the accomplishment of human 
purposes, may be evaluated. The importance of this aim is very great for 
agriculture and productive forestry, and it is not less important for the 
fundamental principles of ecology. The above discussion presents one of 
the first serious attempts to compare the plant-producing powers of several 
climates by means of numerical indices. 



21 Fassig, O. L. The period of safe plant growth in Maryland and Delaware. Monthly Weather Rev. 42: 
152-158. 1914. 



Climatic Conditions of Maryland 405 

GENERAL CONCLUSION 

The results and suggestions attained by the study here reported leave the 
problem of agricultural or ecological climatology still very far from solved, 
but the purpose of this investigation has been achieved if some of the more 
fundamental considerations that must be taken into account in this sort of 
inquiry have been emphasized. The main points brought out are summar- 
ized in the Abstract at the beginning of this paper and do not require repeti- 
tion here. It is clear that this aspect of climatological science required other 
measures and other methods of treatment than those thus far developed by 
meteorological climatologists, and that much physiological knowledge must 
be built into the structure of the new science. It appears that the use of 
standard plants, in some such way as the soy-bean plants were used in this 
investigation, and the avoiding of the immense complications due to soil 
conditions when the same soil is not employed in all cases, will lead to progress 
in this exceedingly difficult but both fundamentally and practically important 
field of human advancement. If the relations that hold between climatic 
conditions and plant growth are to be really understood it will be necessary 
for the climatological student to interest himself in plant physiology in no 
merely superficial way, and it will be necessary for much of the science of 
climatology, as it is now represented in the literature, to be very lightly 
stressed. The point that seems in need of emphasis is that this new aspect 
of climatology (or of ecology) will have to deal with climatic conditions as 
they affect plants; it will not need to give main attention to climatic fluctua- 
tions and differences per se, nor to the meteorological, physical and astronomi- 
cal reasons for their occurrence. 



VITA 

The writer was bom December 26, 1888, at Baltimore, Maryland. He 
entered the Baltimore Polytechnic Institute in 1903, being graduated in 
1907. During the year 1908-1909 be taught in the public schools of Balti- 
more. In 1909 he entered the Collegiate Department of the Johns Hopkins 
University, receiving the degree of Bachelor of Arts in June, 1913. During 
the years 1914-1917 he attended the Johns Hopkins University as a graduate 
student in Plant Physiology, Physical Chemistry and Botany. He was 
engaged in research for the Maryland State Weather Service during the 
year 1915-1916, and carried on research for the U. S. Forest Service at the 
Utah Experiment Station of the Forest Service during the summer of 1916. 



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